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SAS Formulation stoneware WOPL Plasticity Green Strength particle distribution paxking density

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#1 glazenerd

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Posted 17 December 2016 - 10:19 PM

 

Moved from:       http://community.cer...               Copyrighted materials: TJA Oct. 2016

 

 

Introduction of SAS Formulation - hypothesis

 

The industry standard has been formulation based on particle size distribution (PSD), which includes density packing. The Zameck article proposed formulation based on the PSD principle; which included emphasis on density packing. However, particle size is a measurement that only determines the plane (face) of a particle, but does not include the depth. A sugar cube represents a perfectly square particle: which has the same plane and depth equal. Clays however can have the same particle size on the face, but vary widely in depth. So a large grain of clay could be represented by a sugar cube shape: but in reality the depth can be anywhere from a thickness of hair up to a normal sugar cube shape. So using PSD as a determining factor for packing density is inaccurate; because it does not include the measurement of depth (platelet.)

 

 

Specific Area Surface (SAS) is:Specific surface area (SAS) is a property of solids defined as the total surface area of a material per unit of mass. The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape.

 

 

The SAS is far more accurate, in that it takes into account both the width (particle size) and the depth (platelet size) of clay. In common calculations: a potter will factor in a 200 mesh clay: but has no idea if that platelet size is  20-50-100-1200 mesh thick. So determining packing density solely on the basis of particle size is inaccurate, because platelet size is not factored. A 200 mesh clay could have a SAS as low as 18, and as high as 28: which this variance alone confirms the inaccuracy of using particle size alone.

 

 

In addition to using the SAS in determining accurate grain size; the SAS will also give the potter insight into the plasticity of the clay of choice.  The higher the SAS goes, the more plastic the clay will be. (Applicable to ball clays and bentones primarily). More importantly, the SAS becomes the basis of formulation because it factors total grain size. So from here, the SAS formulation needs to be applied..

 

Nerd



#2 glazenerd

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Posted 17 December 2016 - 10:24 PM

 New Testing Standard    "water of plasticity"  WOPL

 

The industry WOPL test indicates the amount of water required to make a formulated body of clay into a puggable ball of clay. However, this test is for a blended body, and gives no indication of what an individual clay requires to produce the same goal. Ball clays in particular are mined locally by many potters, and tested as a blend: which really does not give information about the native clay itself other than final effects. In talking to numerous clay mines, and going through  a lot of technical data supplied and classifying that info on spread sheets: I have narrowed down a median value for ball clay.

 

100 grams of native/ or mined ball clay + 35 grams of water = a pliable ball of clay that can be formed.

- if it is still tacky: that means it requires less water, which also means it has larger platelet size: which equates to less plasticity.

- if it is still dry"ish": and needs more water: that means it has smaller platelet size: which equates to higher plasticity.

 

Higher plasticity ball clays typically equal: 1 - 0.50 micron sized particles and (more importantly) platelet size of under 0.65.

Lower plasticity ball clays usually have the same micron particle sizes: but platelet sizes are over 0.75 micron.

 

For reference: bentonite/s hectorite/s, smectorite/s typically have 1 micron or under particle size: but more importantly platelet sizes under 0.40. While particle size does play a role in plasticity; platelet size ultimately determines plasticity.  In the table of values I composed from the various ball clay specs: 35 represented what the clay mines deemed "plastic." Those  requiring less, were deemed less plastic, and those requiring more- more plastic. Obviously potters know bentonite will soak up lots of water: which just further demonstrates the platelet size equating to plasticity.

 

*Obviously there will always be exceptions: but this makes for a good general rule.

 

The other primary distinction becomes particle size: kaolins typically start at 1 microns and up. While ball clays generally run 1 microns and under.(Both have low % of microns up to 5.)

The other distinction was PPM (parts per million) of carbons.  Kaolins run below 100 ppm, and ball clay runs over. If a ball clay runs lower; then it is also classified as kaolinitic. Ball clays run much higher in sulfur content, kaolins run very low: if any.

 

Nerd

 

Test example: 100 grams OM4 ball clay with 35 grams water = pliable ball. Mix, wedge in your hand a few minutes. Does not stick, not tacky, can be shaped easily.



#3 glazenerd

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Posted 17 December 2016 - 10:26 PM

Additional information / study:  Ball clay/s  PSD: variations of 5.0 / 1.0 / 0.50 microns    (platelet in microns)

 

WOPL  (water of plasticity) linear          Platelet size              Shrinkage ^3

 

33                                                           0.73                            5.8

33                                                           0.67                            6.6

36                                                           0.61                            7.3

38                                                           0.67                            8.5

38                                                           0.37                            7.5

 

Note" WOPL of 35/36 indicates medium plasticity.

 

The two WOPL  levels of 33 have a large difference in the fired shrinkage. The difference between the platelet sizes play a large role in these differences. The other influence is the distribution of particle sizes (PSD).

 

The WOPL level of 36 has a median platelet size of 0.61 microns. As the platelet size decreases, the fired shrinkage increases.

 

The final WOPL level of 38 have a significant difference in platelet sizes. The larger platelet has higher shrinkage, and the smaller platelet has smaller shrinkage:why? The 0.67 platelet size has a much higher percentage of smaller particle sizes, and 0.37 platelet size has a fairly even distribution of particle sizes.

 

While this WOPL test is somewhat subjective: you can use known clay/s such as OM4, or FHC to set a baseline. I ran OM4 in the initial test of 100 grams OM4, with 35 grams of water. After  mixing, I kneaded it in my hand for a few minutes. After that, it was not tacky, not lumpy, and felt smooth to the touch. From this initial test forward: you will know how to gauge other clay/s:  if they are tacky or dry. From the amount of water needed to achieve a smooth, workable clay: you can use the chart to guesstimate the platelet sizes, and have some indication of how much they will shrink by the water required to achieve pliability.

 

Kaolin/s typically have particle sizes ranging from 1.50, up to 2.00.          (median)

Ball clays typically have particle sizes ranging from 1.00 down to 0.40.    ( median)

Bentonites typically have particle sizes under 0.40.

 

Platelet size is the biggest indicator of plasticity; although particle distribution can alter that general rule of thumb. Another indicator that measures both particle size and platelet size into one general number is: "specific surface area." This number takes into account the total surface of one particle of clay: particle size by platelet size = surface area.  A specific area of 18.6 has less plasticity, and a specific area of 25.6 is more plasticity. Common sense; it takes more small grains to fill the same space as one large grain.

 

A clay particle that is 1 micron wide (particle size) by 1 micron tall (platelet size) requires X amount of water to cover all sides. Divide the platelet in half and you have a particle that is 1 micron wide, by 0.50 microns tall (platelet size.), which requires more water to cover because two particles occupy the space of one larger particle..This simple explanation details why high plasticity clays require more water in the WOPL test.  It also explains why higher plasticity clay/s have higher shrinkage rates. BUT_ it also explains why higher plasticity clay/s also provide more mechanical strength: because there is greater surface area.

 

Nerd



#4 glazenerd

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Posted 17 December 2016 - 10:27 PM

Application of the WOPL (water of plasticity) ball clay.

 

You already know from the previous post that the WOPL test verifies the plasticity of ball clay / or native ball clay.  So that information means what?

 

WOPL               Application:

 

33                      Fast setting casting formulation.                              Use low SAS for casting.       High SAS for body

34                      Medium setting casting formulation.  

35                      Slow setting casting / low plasticity throwing             Use a combination to achieve desired goals.

36                      Medium plasticity throwing -stoneware

37                      High plasticity stoneware

38                      High plasticity porcelain.  

38 +                   Bentonites, Hectorites, Macaloid

 

A larger particle with smaller platelet will have similar SAS values as a smaller particle with larger platelets.

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#5 glazenerd

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Posted 17 December 2016 - 10:30 PM

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#6 glazenerd

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Posted 17 December 2016 - 10:30 PM

PART 2:  Converting PSD to SAS

 

25 parts Hawthorne 40m          25 parts x 20 SAS        = 500 SAS                   Cat.1

35 parts Imco 400 200m          35 parts x 21 SAS        = 735 SAS                   Cat.2

17 parts ball clay                      17 parts x 25 SAS        = 425 SAS                   Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.           Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.           Cat.4

1660 SAS divided by 77 parts (clay only) = 21.56 median SAS

 

Hawthorne 40M has 27% particles under 120 mesh; the balance is 120m plus.

120 mesh is the current threshold for particle size incorporated into a melt.

 

sml_gallery_73441_1262_4956.jpg
In this commercial stoneware body: mesh sizes of 30-80 are plainly visible. Fired to 2230F/10 minute hold.
 

Imco 400 is air floated and runs fairly consistent in particle sizes.

Ball clay is sub-micron; distribution variance does not affect melt.

Only the percentage of distribution in Hawthorne 40m needs to be adjusted.

 

25 parts Hawthorne 40m          25 parts x 20 SAS = 500 SAS x 1.27 = 635 SAS       Cat. 1

35 parts Imco 400 200m          35 parts x 21 SAS = 735 SAS                                    Cat.2

17 parts ball clay                      17 parts x 25 SAS = 425 SAS                                   Cat.2

10 parts silica                           17 parts x 7 SAS          reporting value.                    Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                    Cat.4

1795 SAS divided by 77 parts (clay only) = 23.31 median SAS

 

Explanations: Why was Hawthorne multiplied? The large mesh grains have larger platelets (21SAS), but 73% is 120 mesh and higher which will have a wide variance in platelet sizes. There is no definitive data available that breaks down SAS per mesh sizes. So 27% would be the conservative addition to compensate for finer meshes.

 

What does the median formula value of 23.31 SAS mean?

*SAS values range from 18 (coarse) to 28 SAS (ultra-fine). for clay only.

On the SAS scale; 23.31 is slightly over the medium platelet size. In addition, 18SAS represents non plastic and 28 SAS very plastic. SAS 23.31 is then projecting a medium plasticity body. To increase plasticity: by removing some fire clay or 200 mesh intermediate clay and replacing it with 25 SAS ball clay. The other option is to use a ball clay with a 28 SAS value.

 

Edit Note: changes in the SAS values from the original post reflect upgraded information supplied by the mines.

 

Additional formulation limits to be provided as testing verifies results.

The SAS Formula by Tom Anderson (GlazeNerd)  Copyright Nov. 20, 2016

Submitted to Ceramics Monthly & Technical Ceramics.



#7 glazenerd

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Posted 17 December 2016 - 10:31 PM

Read "Green Strength" post below before proceeding.

 

How do you adjust the SAS value when formulating?

 

25 parts Hawthorne 40m          25 parts x 20 SAS = 500 SAS x 1.27 = 635 SAS     Cat.1 

35 parts Imco 400 200m          35 parts x 21 SAS = 735 SAS                                 Cat.2

17 parts ball clay                      17 parts x 25 SAS = 425 SAS                                Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                 Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                 Cat.4

1795 SAS divided by 77 parts (clay only) = 23.31 median SAS

 

This formula from above will be used in this example. Let's say the potter wants to use a higher amount of fire clay because they want to raku or salt fire: and they need a stronger/denser body:

 

30 parts Hawthorne 40m          30 parts x 20 SAS = 600 SAS x 1.27 = 762 SAS      Cat.1

30 parts Imco 400 200m          30 parts x 21 SAS = 630 SAS                                   Cat.2

17 parts ball clay                      17 parts x 25 SAS = 425 SAS                                  Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                   Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                   Cat.4

1817 SAS divided by 77 parts (clay only) = 23.59 median SAS

 

As the percentage of category 1 large particle fire clay increases: the SAS must also increase to compensate for density packing and particle distribution. The original recipe had 25% category one, and this revised recipe has 30%: an increase of 20% by volume. To compensate, ball clay is added at a minimum of 2 to 1 ratio for fire clay. (1part ball clay for every two parts fire clay). The original SAS median value of 23.31 must increase by 10% (2to1) to 25.64. This will be nearly impossible because you are entering into a non-functional body territory. However, it is possible to push the SAS values way up.

 

30 parts Hawthorne 40m          30 parts x 20 SAS = 600 SAS x 1.27 = 762 SAS      Cat.1

25 parts Imco 400 200m          25 parts x 21 SAS = 525 SAS                                   Cat.2

5 parts EPK                             5 parts x 28.56 SAS = 143 SAS                              Cat 2

17 parts ball clay                      17 parts x 28 SAS = 476 SAS                                  Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                   Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                   Cat.4

1906 SAS dividided by 77 parts (clay only) = 24.75 median SAS.

 

By lowering the Imco 400 to 25 parts from 30 parts, and replacing it with 5 parts of EPK: additional SAS values are realized. The same amount of ball clay was used: but the 28 SAS value indicates a different ball clay with much finer platelet size. Although the final SAS projection was not reached: the packing density was greatly increased. This body will be more plastic, will have increased green strength; and be far less susceptible to stresses of raku or salt firing.

 

Generally speaking, once category one fire clays cross the 25% recipe range: the ability to supply sub micron fines to close porosity becomes increasingly challenging.

30 35m
 
The large particles of the fire clay become so heavily populated, it becomes nearly impossible to supply enough fines to encapsulate them. As you can see in this 30% category one sample: large grain clay particles rest against each other.

 

Nerd



#8 glazenerd

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Posted 17 December 2016 - 10:31 PM

The SAS Formulation system requires that you divide the formula into four categories:

 

1. Large particle fire clay  20-120 mesh

2. Intermediate clay: 120-200  200 - 325 mesh    **Cone 6 needs finer meshes than cone 10.

3. Silica - reporting value.

4. Feldspar. - reporting value.

 

Only category one needs to be adjusted for SAS values: see the example above. Category two will change according to the percentages of each type of clay you use. All other clay/s go into this category, as long as they are not large particle fire clays.

 

Silica and feldspar are reporting values: however there will be some differences between a cone 10 and cone 6-5 bodies. A 200 mesh silica will have a lower SAS value (5-6 SAS), and a 325 mesh silica will have a SAS value of 7-9. By making this notation in your reporting values, you are notating mesh sizes. The same is applicable for feldspar. In a cone 5-6 body; higher SAS valued feldspar will provide a quicker melt because of the overall SAS value.

 

You will also need to increase the median SAS value when formulating for cone 5-6 bodies. The higher SAS value equates to finer mesh and platelet sizes: which will lead to a quicker and more complete melt.

Nerd



#9 glazenerd

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Posted 17 December 2016 - 10:32 PM

Green Strength Properties  (Ball Clay)   Educational Post

 

Ball Clay           MOR          SAS          PSD           Al2O3       WOPL

 

OH #5              475            18.9          0.67            29.01         33

OH M23           700            23.3          0.67            25.10         38

M44                 770            25.6          0.73            27.50         33

Kaolin --------------------------------------------------------------------------

EPK:                300            28.52        1.36            37.46         26

Gold Art 200m                   20.00                           27.30                   no other data available.

Imco 400 200m                  21.00                           34.40                   .

 

MOR (Modulus of Rupture)   SAS (Specific Area Surface)  PSD (Median Particle Size)   Al2O3 (Alumina)   WOPL (Water of Plasticity)

 

 MOR is actually tested as "the modulus of flexure", meaning how much the sample will bend before it breaks. Modulus of Rupture has little importance in the pottery world: but modulus of flexure does: because we call that green strength. It is common knowledge that porcelain has low green strength, and stoneware has much higher green strength. The difference is directly related to particle size and platelet size (SAS). While ball clay is added to increase plasticity: those additions automatically increase green strength because the packing density has been increased as well.

 

As you can see by first glance: the MOR values of EPK (kaolin) compared to the ball clay examples is vastly different. The SAS is higher than the ball clay/s shown: but the PSD is double to triple the size of ball clay. Packing density is used in stoneware formulation to define how closely together the various clay particles are aligned: however particle size plays a major role in green strength. EPK is one of the smaller particle sized kaolins, which also has a very thin platelet size of 28.52: which gives it the title of being "plastic." It is plastic as kaolin goes, but not so plastic in relation to ball clay.

 

The other point of interest is the correlation between WOPL values and MOR values. As the WOPL values increase from 33 to 38: so does the MOR values. For this reason I suggest to those mining their own clays to use the WOPL test to estimate particle size and platelet size. The larger the WOPL value is, the more plasticity the native clay has: and the finer the PSD/SAS will be.

 

There seems to be one exception to the general rule of WOPL = green strength- why? M23 has a WOPL of 38 and a MOR of 700. M44 has a WOPL of 33, and a MOR of 770- why? If you look at the PSD and SAS values you will notice the explanation. Both have the same relative PSD value, but M44 has a much thinner platelet as indicated by a SAS of 25.6

 

Note:A larger particle with smaller platelet will have similar SAS values as a smaller particle with larger platelets.

 

In determining which ball clay to use in a stoneware body: MOR, SAS, and WOPL values need to be examined. While a WOPL of 33-38 is plastic to highly plastic at 38: SAS values will ultimately determine plasticity. The smaller the PSD and the higher the SAS values are: the more plasticity there will be. More plasticity also equates to green strength: and also equates to higher packing density values. The other feature in selecting a ball clay is the alumina content. Stoneware requires a higher amount of alumina than a porcelain body: because the alumina is an important component in mullite production. You might wish to sacrifice some plasticity in order to increase alumina molarity.

 

These are the fine point explanations of why I introduced the SAS Formulation standard.  As you formulate a stoneware body, and compute the final SAS value: it gives you a definitive value of packing density, green strength, and plasticity. These are all inner-related values. A final SAS formulated body that runs between 23.50 up to 24.00 SAS has a good distribution of particle sizes. To keep it in that range you either have to rearrange the amount of large particle fire clay, intermediate clay, or use a ball clay that has higher SAS values.  The most compelling evidence of how much ultra fine particles are in the body is how much it creams up when you throw it.

 

Nerd



#10 glazenerd

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Posted 17 December 2016 - 10:33 PM

Adjusting the SAS Formula - Part 2 - FOR YOUR SPECIFIC PLASTICITY PREFERENCE.

 

 

The recipes given as examples above, include the amount of ball clay I prefer to achieve the plasticity that I like. There is however a minimum  amount of fines required to encapsulate the large particles of fire clay.

 

10 1200m
 
Silicon Carbide was used as a tracer in this blend experiment to establish a visible marker. The large particle in the center of the picture is completely encapsulated with SiC tracers. In this particular blend experiment; 15% of large particle (category one) fire clay was used. While the interpretation of data is somewhat subjective: some will view it as overly populated, others will view it as acceptable. The baseline objective is to formulate a body that is vitrified (encapsulated) for functional use. While porcelain does become fully vitrified: stoneware is acceptable with absorption around one percent. So the primary goal of SAS formulation is to provide enough sub-micron particles to completely encapsulate large mesh fire clay: coupled with the correct Si/Al levels, and the adequate flux molarity. The end goal being functional use. However, the SAS formula can be used for non-functional bodies to create the strongest, densest bodies achievable.
 
The minimum sub micron fines required for SAS formulation is 0.75% of category one fire clay.
EX> 15% category one fire clay times 0.75 = 11.25  Round up to 12   ( Clay formula does not require the precision of glaze.)
 
The easiest way to establish a median SAS value that reflects your personal preferences for plasticity, is to convert one of your favorite recipes to the SAS formula. The final median SAS value given after you have adjusted for large particle fire clay is your preferred range. However, if that median value falls below the minimum required for functional use: then adjustments need to be made.
 

Minimum sub- micron particles

15 parts Hawthorne 40m          15 parts x 20 SAS = 340 SAS x 1.27 = 381 SAS   Cat. 1

*Minimum fines required: 15 parts fireclay x 0.75= 11.25 round up to 12  

12 parts ball clay                      12 parts x 25 SAS = 300 SAS                                 Cat.2

48 parts Imco 400 200m          48 parts x 21 SAS = 1008 SAS                                Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                  Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                  Cat.4

1689 SAS divided by 77 parts (clay only) = 21.94 median SAS

 

This would be a lower plasticity body, heavy body; suitable for a cone 10 firing. It would be suspect however for a cone 6 application. The high population of 200 mesh category 2 clay would be suspect if flux levels were too low: or if it was fired incorrectly. For cone 6, I would like to see more fines.

 

15 parts Hawthorne 40m          15 parts x 20 SAS = 340 SAS x 1.27 = 381 SAS        Cat.1

*Minimum fines required: 15 parts fireclay x 0.75= 11.25 round up to 12   

12 parts ball clay                      12 parts x 25 SAS = 300 SAS                                    Cat.2

38 parts Imco 400 200m          38 parts x 21 SAS = 798 SAS                                  Cat.2

10 parts EPK                           10 parts x 28.56 = 286 SAS                                      Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                     Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                     Cat.4

1765 SAS divided by 77 parts (clay only) = 22.92 median SAS

 

Or if you like high plasticity bodies:

 

15 parts Hawthorne 40m          15 parts x 20 SAS = 340 SAS x 1.27 = 381 SAS      Cat. 1

*Minimum fines required: 15 parts fireclay x 0.75= 11.25 round up to 12   

18 parts ball clay                      18 parts x 25 SAS = 450 SAS                                 Cat.2

  • Plus 6 additional ball clay for plasticity:

32 parts Imco 400 200m          32 parts x 21 SAS = 672 SAS                                  Cat.2

10 parts EPK                           10 parts x 28.56 = 286 SAS                                     Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                  Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                  Cat.4

1789 SAS divided by 77 parts (clay only) = 23.23 median SAS

 

Want more plasticity: then use a higher SAS rated ball clay. Remember, as the SAS value rises: the PSD becomes finer, and the plasticity increases.

 

15 parts Hawthorne 40m          15 parts x 20 SAS = 340 SAS x 1.27 = 381 SAS     Cat. 1

*Minimum fines required: 15 parts fireclay x 0.75= 11.25 round up to 12   

18 parts ball clay                      18 parts x 28 SAS = 504 SAS                                  Cat.2

•           Plus 6 additional ball clay for plasticity:

32 parts Imco 400 200m          32 parts x 21 SAS = 672 SAS                                   Cat.2

10 parts EPK                           10 parts x 28.56 = 286 SAS                                      Cat.2

10 parts silica                           10 parts x 7 SAS          reporting value.                   Cat.3

13 feldspar                               13 parts x 9 SAS          reporting value.                   Cat.4

1843 SAS divided by 77 parts (clay only) = 23.93 median SAS

 

This body works fine using a SAS 25 ball clay, if you want more plasticity use SAS 28 ball clay. If you plan  on making grog additions: you are adding more large particles. Compensate by using only SAS 28 clay, or increase the EPK slightly, and subtract that change from the  Imco 400.

 

As you can see by the final median SAS values: formulation falls into the mid 20 range:  22.00 up to 24.00 SAS. This range is acceptable for most bodies designed for functional use. However, after absorption testing and weep testing: if a problem exists: then increase the sub micron fines; either by increased ball clay, finer ball caly (higher SAS), or additions of  EPK (SAS 28.56)

 

Nerd



#11 glazenerd

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Posted 17 December 2016 - 10:34 PM

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#12 glazenerd

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Posted 17 December 2016 - 10:36 PM

Educational Post moved from another thread:

 

Posted Today, 09:27 AM

Why aged clay is smoother?

 

Stoneware in particular changes characteristics over time, but all clays do to some degree. The common thought is because of bacterial growth (fungus/mold, etc. Bacterial growth is a reflection of how much organics is in the clay itself (ball clay primarily). If you are getting a lot of bacterial growth on your clay: it indicates high levels of organics: which means you need to bisq slightly higher, or with a hold to burn them off completely.

 

The "aged" effect is actually due to the clay particle itself. On a molecular level, clay particles look like Swiss cheese: porous. When you first mix clay it is all soft and gooey because the water is binding the clay particles together. However, when you bend or twist it: it has the tendency to snap because it is "short." As time passes: molecular H20 penetrates into the molecular pores of the clay: and then the full plasticity level of the clay is obtained. (WOPL= water of plasticity). You will also notice a change in consistency from very soft when first pugged, to various degrees of firmness as time passes. The clay has not lost moisture content, it has absorbed moisture content. Which is also the reason blunged clay is more plastic than pugged clay: because mechanical forces speed up the process of absorption.

 

Normally within 30 days there is a marked difference, which improves over the next 90-120 days. After about 6-8 months, the process begins to reverse because the clay is actually starting to loose water: dehydration. Absorbing water is hydration, losing water is de (loss of).

 

Nerd



#13 glazenerd

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Posted 17 December 2016 - 10:37 PM

The issue of drying, hardening, and water retention in reclaimed clay is a different issue. It comes back to the WOPL (water of plasticity) principle of ball clay. It is the ball clay in stoneware (primarily) that governs water absorption: a ball clay with 33WOPL retains less water, and a ball clay with 38WOPL retains much more. So the effect, what you are speaking of is determined by the amount of ball clay (ultra-fines) that is lost.

 

How do you determine the level of ball clay the recipe had to begin with?    The Cream Test

 

When you throw the original clay: how much cream comes up and on your hands?

 

Coats just the inside of your palms and oozes through your fingers over time.... lower levels.

Coats your palms, and oozes; have to clean a few time while throwing.... mid levels

Oozes quickly and constantly cleaning off hands....... high levels.

 

Most of the reclaimed scraps is from trimming; which has been stripped of the fines; which includes ball clay, silica, and feldspar. That would alter the properties of recycled clay: because it is the ball clay primarily that holds moisture in a clay body.

 

The fix:  blend 80% ball clay (Om4 or FHC), 10% silica, and 10% feldspar. Add 1 cup (dry) per gallon of slurry. The testing comes when you throw it after it has been reclaimed: how much cream comes up when you throw? Adjust to suit your taste.

 

Nerd



#14 glazenerd

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Posted 17 December 2016 - 10:37 PM

Fired Brightness

 

Whiteness and brightness only becomes an issue for stoneware when it is used for functional ware. Obviously the color of the clay can alter the glaze color: so whiteness in stoneware in meant to duplicate the whiteness of porcelain for glaze color and brightness. Most ball clay do not give the amount of carbon or sulfur content: so you have to gauge how bright the fired result will be using other values.

 

Clay                 Color                   Alumina             FE            LOI

 

M28                Lt. Brown             25.60                0.87           9.01

No.5               White                    29.01                0.88           9.79

M33                Dark Brown          25.01                1.71           17.0

No1 (glaze)     White                   29.37                0.87           9.97

Taylor              Lt. Tan                 26.40                1.06           9.98 

C& C               Off white              26.29                0.92           10.40           

 

(LOI) Loss on Ignition: indicates carbonaceous materials that burn off when the kiln hits red heat levels.

 

The color of the clay does give some indication: but it cannot be relied on entirely. The carbonaceous material in the clay burns off: which is reflected in the LOI values. The higher the value goes, the more carbons in the clay. M28 has an LOI of 9.01 which has a color of Lt. Brown: but C&C is white and has an LOI of 10.40. So color alone is not an exact indication of carbon content. 

 

When ball clay begins to cross the 25.00 alumina mark: it begins to be classified as kaolinitic pending carbon content. No5 and No.1 both have over 29.00 alumina and are classified as kaolinitic ball clay, coupled with lower LOI values. M28 is also designated as kaolinitic having only 25.60 alumina: but very low LOI values. The other indication of fired brightness is iron levels. M33 has a high LOI value, and also a high amount of Iron (FE): the high carbon/iron content is reflected in its color. Even if a ball clay has a lower LOI value, but over 1.00 iron: it can still add color to the final clay body. To achieve high whiteness/brightness in a clay body: the ball clay must have lower LOI values, and iron should not be over 1.00%. 

 

Note: EPK and #6 Tile kaolin are often found and used in white stoneware bodies for functional use. EPK will add high SAS value, in addition to some plasticity. #6 Tile provides a high alumina content, and a larger particle/platelet size for added body. They can be used separately or in conjunction with each other. It is not uncommon to find either or both in amounts up to 25% of the total recipe. The addition of kaolin/s with ball clay is the basis of a 50/50 porcelain body. When fire clay additions are made up to 15%, the property is often called "tooth."

 

In regards to using ball clay for glaze additions: No 1 (known as SPG1) is a much better choice than C&C ball clay. It has a lower amount of iron, and much higher amounts of alumina. Alumina is an important component for glaze because it adds strength to the glaze: which helps guard against cutlery marks. In addition, the lower LOI value will also help lower the overall COE of the glaze.

 

As you begin to learn the various clay values: it will make selecting ball clay easier. Of the ball clays listed: Taylor is the optium overall choice because is has higher alumina, and lower LOI values. More importantly, it has a WOPL value of 38: meaning it is a highly plastic, kaolinitic ball clay. In addition, it has an ultra fine sub micron particle size of 0.31: one of the smallest of all ball clays.

 

Carbon Coring: if you have ongoing problems with carbon coring after you have adjusted your firing schedule: using this section for whiteness will reduce the carbonaceous materials in your recipe.

 

Nerd    



#15 glazenerd

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Posted 17 December 2016 - 10:37 PM

Understanding plasticity,  memory, and general clay properties.

 

For those interested in the chemistry of clay:

 

 

https://en.wikipedia...change_capacity

CEC (cation exchange capacity) is the exact mechanism behind clay plasticity. It is common knowledge among clay junkies that the particle charge determines plasticity. A negative charge means the particles are repelling each other: which is commonly expressed in the clay arts as; "sliding by each other." A neutral or positive charge then means clay particles are attracted to each other: which is the basis of memory. Mechanical forces (throwing/rolling) distorts the grain boundaries formed by a positive charge: but the positive charge attracts those distorted particles: drawing them back into a positively charged particle alignment. 

 

Towards the bottom of the article posted is the "standard values" for clays and soils (CEC). It is measuring the properties of clay types to readily exchange cations in the presence of water.

Quote

 

Kaolinite 3–15 Halloysite 2H2O 5–10 Halloysite 4H2O 40–50 Montmorillonite-group 70–100 Illite 10–40 Vermiculite 100–150

As you can see: kaolin and halloysite (new zealand kaolin) have very low CEC values, while montmorillonite (bentonite) and vermiculite have very high cation exchange capacities. (CEC) Vermiculite is classified as a 2:1 clay structure: which is the same structure classification given to ball clay. In simple terms: a 2:1 clay structure has two inner platelet structures, and one exterior platelet structure. Kaolin is a 1;1 structure: meaning is only has a surface platelet, with no interior structures. This is the important distinction between porcelain and stoneware: because the clay variety dictates how the formulated body reacts/acts. A 1:1 clay structure (kaolin) can only absorb water onto its exterior platelet, while a 2:1 clay structure can absorb water onto the face of the platelet, but also into its interior structure. This particle structure then effects the clay type ability to A. carry its own electrostatic charge, or B. conduct/transmit a electrostatic charge.

 

Kaolin has a 1:1 particle structure: so it can only absorb water onto the face of the platelet. Secondly, it has a neutral electrostatic charge (polarity), so that neutral charge means it has no ability to influence or transmit that charge to adjacent particles or other elements mixed with it. This property means kaolin will therefore be readily influenced by whatever electrostatic charges of the materials mixed with it: and that charge will only be carried onto the face of the platelet. Sodium and potassium both have strong positive electrostatic positive charges: sodium moreso than potassium. So when flux is added; the positive charge of those fluxes are carried on the platelets of the kaolin. A positive charge then A. greatly reduces plasticity because a negative charge is required for the particles to slide past each other. B. increases memory properties because the positive charge attracts the adjoining particles when mechanical forces separate or distort them.

 

Ball clay has a 2:1 particle structure; which can absorb water onto its exterior face, but also into it secondary platelet structures. Ball clay has a naturally occurring negative charge: which is why ball clay is so plastic. The negative electrostatic polarity resides both on its platelet face, but also inside its interior structures. In this case, when positively charged fluxes are added: while the positive charge effects the platelet face: the interior structure retains its negative charge. The negative charge is the over-riding polarity in the clay body (stoneware) and its plasticity is preserved. In addition, the negative charge is the over-riding influence in the body: effectively eliminating the memory caused by a positively charged clay body.

 

The 1:1 structure of kaolin limits absorption of water; being bond to its platelet face only. This also determines it's drying time, because the platelet surface shed water at an accelerated rate: causing a much more rapid drying time. Ball clay being a 2:1 clay structure absorbs water onto its platelet face and into the secondary inner structures. This results in a much longer drying time because the water absorbed into the interior secondary structure takes much longer to dry. The absorption of water into its secondary structure is what also determines how soft or firm a stoneware body feels to throw pending on the ball clay type, the percentages used, and the amount of fire clay added. What feels like :mass", is actually the additional water absorbed into the interior structures.

 

The other key in differentiating kaolin and ball clay is alumina levels. Kaolin have much higher percentages of alumina compared to ball clay. While it is widely accepted that kaolin and ball clay are both aluminosilicates separated only by the amount of carbons: that is not a true assessment. Alumina is a metal oxide that is solely responsible for the lack of a negative charge in kaolin. Alumina typically has a much higher positive charge (up to 3x) that of silica: the other major component in clay. The lack of alumina in ball clay is what effects its valance polarity: being negatively charged giving its plasticity. The  higher levels of alumina in kaolin is what causes it to have a neutral electrostatic charge, causing it to have much less plasticity; and much more memory. While kaolin and ball are both clays: it is the alumina levels that makes the distinguishable differences between them.

 

Stoneware being mainly comprised of low alumina ball and fire clay does not require additions of negatively charged ball clay or polymers to produce its plasticity. Porcelain being primarily kaolin, requires negatively charged ball clay or polymers to create its plasticity. In addition, the structure of the clay also determines how much water the clay particles will hold: which also determines how fast it will dry.

 

Nerd



#16 glazenerd

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Posted 17 December 2016 - 10:38 PM

Firing cone 5-6 stoneware bodies.

 

Firing to cone 10 achieves enough heat work, and requires more time: the combination usually resolves any issues related to lower cone firing schedules. In addition, iron becomes a much more active flux at cone 10: adding to the maturity/vitrification of the body.

 

Stoneware clay relies primarily on potassium as a flux: because many of the clays involved have naturally occurring levels of potassium to start with. Potassium is a late bloomer: meaning it begins to melt at a higher temperature than sodium: the common flux in porcelain. The later melt also means it is still off gassing, after sodium would normally have stopped.

 

The other issue is mullite formation in a stoneware body. Metakaolin changes to spinel around 2050F: and spinel is the basis of mullite. Spinel / mullite is formed from alumina and silicate. At 2180F, potassium has fully fluxed, and mullite production is at its peak. So if you are ramping through the 2050F to 2180F range too quickly: you are actually hindering the clays ability to mature/vitrify. In addition, you are also blowing through the period when off gassing is at its peak.

 

To Remedy (firing bisq)

 

Use your normal cycle to 2050F

130F from 2050F to 2190F- with extended hold.

OR

130F to 2230F- with much shorter hold.

 

** Kiln size will determine hold cycle.

 

Using this ramp cycle will mature the clay: which is much more important when firing functional ware. In addition, the incidence of pin-holing will be greatly reduced: if not completely resolved. If you are still getting minor pin hole issues: either slow the top ramp to 125F an hour, or add 10 minutes to your peak hold.

 

Nerd



#17 glazenerd

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Posted 17 December 2016 - 11:59 PM

To answer some questions from PM posts:

 

SAS formulation defines mesh sizes: it does not replace mesh size. The PSD standard is still being used, and packing density is being defined as a SAS value. As you go back over the revised post; you will notice an initial SAS value of 21.56, and a final SAS value of 23.31. So in regards to packing density: 21.56 is packed looser because lower values reflect coarser platelets. As you move up the scale to 23.31: you have increased packing density because platelet sizes are finer.

 

The higher the final SAS value: the more plastic the body will be. In addition, higher SAS values also reflect increased green strength. Although I am still working on the chemistry side of formulation: you will also find that higher SAS values will also reflect higher alumina molarity. For those using native clays: use the WOPL test to determine plasticity. Once you determine a WOPL value you can make certain assumptions based on the results. Higher WOPL values indicate finer sub micron ball clay content, which also indicates higher alumina content.

 

At some point a hard Si/AL ratio needs to be established for cone 6 and cone 10 firings. There needs to be a targeted number for several reasons: primarily because mullite is formed from alumina/silicate. So there has to be a relational balance between the two. In addition, excess silica becomes very problematic in stoneware bodies due to dunting (cristobalite) The remedy usually means the addition of fluxes to counter excess silica: but that can lead to bloating. So definable limits need to be established on the chemistry side.

 

 

Now you can post: I have completed/compiled the information.

 

By the way, one year ago I joined this forum.

 

Nerd

 

Adjusting SAS formula part 2   added 12/19/2016



#18 oldlady

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Posted 18 December 2016 - 09:16 AM

and we are glad you did. :)


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#19 preeta

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Posted 18 December 2016 - 12:55 PM

thanks Tom. Lots to chew on for a while for me. exciting stuff. THANKS!!!!


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#20 What?

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Posted 18 December 2016 - 10:32 PM

I really like the logic and formulation of a of your SAS. If I was mixing my own clay I would be all over this. I have for years added OM4 to my reclaim (stoneware). I Keep a bowl of it on the wedging table and wedge the clay and set for a month and wedge again a use. Works great. I probably don't even use cup per 25lbs.







Also tagged with one or more of these keywords: SAS Formulation, stoneware, WOPL, Plasticity, Green Strength, particle distribution, paxking density

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