Surface Crack In Red Clay ( Terracotta)

[Gokul]

Hi, I have this problem of cracking or clay splitting when throwing in the wheel, pls find the attached picture and also, after firing at cone 08, the clay has a lot of surface cracks and sometimes structural cracks pls find the attached pic, pls advise why this is happening and how to avoid this in future. Thanks.

[Callie Beller Diesel]

Hi and welcome to the forum!

Judging from that first shot especially, you're leaving entirely too much water in the bowls while you're throwing them, and the clay looks overworked or too wet. It looks like your clay has a low tolerance for water in the bottom while you throw, and you need to clear it out more. The second picture looks like not enough compression on the bottom, and perhaps leaving a skim of water on the inside.

In the third picture, did you leave a film of throwing slip on the inside of that bowl or line it with a layer of Terra sig? It looks like there's some kind of layer of fine clay particles on the surface, but it's not something I can gauge properly from a photo. Do those cracks go all the way through the pot, or are they just on the surface?

The fourth image I can see the cracks are structural. It makes me wonder about how damp your pots are when they go in the kiln, and how fast your bisque is. Do you control your own firings, or is someone else doing it for you? Do you live in a humid area?

Also, what kind of clay are you using? You mention terra cotta, but which brand or recipe is it? Do you throw with a rib at all?

[yappystudent]

Just a couple guesses but perhaps the clay was over-wedged before you began to throw, AKA over-worked so the clay particles are separating. Possibly your clay has a low elasticity. Possibly the drying process was rushed. 

[curt]

Google Hamer and Hamer on dunting.  Could be you are not bisque firing hot enough.

[Gokul]

On 4/10/2018 at 9:23 PM, Callie Beller Diesel said:

Hi and welcome to the forum!

Judging from that first shot especially, you're leaving entirely too much water in the bowls while you're throwing them, and the clay looks overworked or too wet. It looks like your clay has a low tolerance for water in the bottom while you throw, and you need to clear it out more. The second picture looks like not enough compression on the bottom, and perhaps leaving a skim of water on the inside.

In the third picture, did you leave a film of throwing slip on the inside of that bowl or line it with a layer of Terra sig? It looks like there's some kind of layer of fine clay particles on the surface, but it's not something I can gauge properly from a photo. Do those cracks go all the way through the pot, or are they just on the surface?

The fourth image I can see the cracks are structural. It makes me wonder about how damp your pots are when they go in the kiln, and how fast your bisque is. Do you control your own firings, or is someone else doing it for you? Do you live in a humid area?

Also, what kind of clay are you using? You mention terra cotta, but which brand or recipe is it? Do you throw with a rib at all?

Thanks for the reply Diesel, We havent use any slip on the clay and we keep the pieces for firing only after its completely dry and sometimes the cracks are just the surface cracks and sometimes its structural cracks, And yes we are doing our own firing with skutt kiln and the pieces are fired at cone 08 in medium fire setting which is I guess its 150 D / hr.

Basically I m from India and the clay which we are getting locally in powder form, we process and filter it with 100# (sieve) and pug it and we are using it and selling it all over india for the studio potters and for the schools all over india., and yes the clay seems like lacking of plasticity and we are adding 2% of bentonite to it to make sure the clay has fine plastic particles and as well as the plasticity.

[Mark C.]

The clay looks short as we say and needs to be more plastic. It looks like to much water is being used too throw it. This will also make it crack more. I would add more materials to make it more plastic and  age it and use less water when working with it.

[glazenerd]

After studying these pictures for a month, I have narrowed down my short list.

Picture 1 indicates a short clay, more specifically a red clay with high levels of sesquioxides. This type of clay resists water absorption, as the standing water indicates. If this clay is used as 50% of the recipe! you will need to increase plasticizers by a fair amount.

Picture 3 is the most telling to me. I can see three distinct and seperate issues that add up to serious cracks. Again, all three trace back to the sesquioxides; specifically FeO3.  The rim is the usual terra cotta, but the base is turning red. This would mean high iron content, combined with iron reduction. (Yes, I know you fired in oxidation.)

The sulfides in the clay were heated too quickly, thereby reducing the iron. The iron level is high enough to cause brittleness as well, part of the explanation for cracking. The third problem, caused by high iron as well: is cristabolite inversion. Many potters know about quartz inversion, but cristabolite inversion causes 2-3 times the stress on fired pieces. Any potter who has heard "pinging" sounds coming from their kiln while cooling (200C / 392F) knows about cristabolite inversion when they opened it to find crazing issues. In your case you have multiple issues happening at the same time, but all stemming from high iron content.

The red clay is causing the problem. It has either changed chemically as they dig through the mine, or your recipe addition is too high. Secondly, this same red clay has high levels of sesquioxides that require much more plasticizers. The black spots on the rim in picture 2 looks like possible coring issues: also iron (disulfide) related. Finally, high iron can also elevate cristabolite formation: which likewise makes the fired piece brittle and susceptible to stress cracks.

My best educated guess from looking at pictures. I would take a hammer to the bowl, my guess is you will find coring issues.

Tom

[curt]

Edit:  To my eye the picture below, from Hamer and Hamer Dictionary of Ceramics, under pretty much sums up your cracking problem.   Bisquing only to Cone 08 (954 Celsius) seems very low temperature to me, and consistent with what is said below may be part of your problem.  Not saying the things mentioned by others above are not also part of the problem, but this to me is the main issue.  Solution is bisque fire higher (1000 or 1030 Celsius, and cool slower from the bisque, and don’t stack pots inside of each other in the bisque.

also, is the top picture you provided terra cotta or a white stoneware body?  It seems to have the same cracking problem, and if it is stoneware this would suggest your problem is not the clay body.

 

[glazenerd]

Gokul:

just finished reading an abstract from Dr. Sutharsan: PHD physics from Annamial University. 

Apparently India has its own unique clay chemistry due to the subtropical weathering of silty clays.

Your red clay is laterite: very high in iron and alumina: higher than those found elsewhere. Secondly, the iron content comes from hematite, and not pyrite: which is good news in one sense, bad in another.

Dr. Sutharsan ran thermal analysis on your red clay: quartz inversion occurs at 560C, and more importantly was his report of an exothermic reaction at 991C. He reports this temperature as the spinel/ mullite conversion temperature. In plain pottery terms: the temperature at which vitrification begins. Bisq firing to this temperature would be problematic because the porosity of the body is beginning to close: so shivering issues could possibly occur. The peak firing temperature of your red clay is 1100C.

particle distribution reported as 45 um to 90 um.   Rather large for red clay.

Your ball clay is of the silty alluvial variety; also much higher in alumina than found elsewhere. Oddly enough many deposits reported no calcium content: which plays a role in plasticity.

After getting some background on your clay types: I would start by doubling your plasticizer additions. (4% bentonite) Secondly, you will not be able to follow the typical firing cycles and peak temperatures used commonly elsewhere because your clay varieties have their own unique chemistry, not found elsewhere. Lastly, Dr. Sutharsan specifically recommended the red clay not be used in ceramics due to its high iron, alumina, and particle sizes. I believe you can use it, but you will have to develop your own customized formulation criteria to overcome its properties. Other articles reported spikes in the iron content upwards of 8%, which I suspect is what has happened here.

Tom

 

[Min]

@Gokul, have you run through some clay testing procedures? My first thought is you are experiencing water splitting, the split in the wet pot on the wheel in your first photo appears to show this. Simple to test for water splitting, about half way down this link there is an explanation of how to go about this. Have you done a sieve analysis? 

[Gokul]

18 hours ago, glazenerd said:

Gokul:

just finished reading an abstract from Dr. Sutharsan: PHD physics from Annamial University. 

Apparently India has its own unique clay chemistry due to the subtropical weathering of silty clays.

Your red clay is laterite: very high in iron and alumina: higher than those found elsewhere. Secondly, the iron content comes from hematite, and not pyrite: which is good news in one sense, bad in another.

Dr. Sutharsan ran thermal analysis on your red clay: quartz inversion occurs at 560C, and more importantly was his report of an exothermic reaction at 991C. He reports this temperature as the spinel/ mullite conversion temperature. In plain pottery terms: the temperature at which vitrification begins. Bisq firing to this temperature would be problematic because the porosity of the body is beginning to close: so shivering issues could possibly occur. The peak firing temperature of your red clay is 1100C.

particle distribution reported as 45 um to 90 um.   Rather large for red clay.

Your ball clay is of the silty alluvial variety; also much higher in alumina than found elsewhere. Oddly enough many deposits reported no calcium content: which plays a role in plasticity.

After getting some background on your clay types: I would start by doubling your plasticizer additions. (4% bentonite) Secondly, you will not be able to follow the typical firing cycles and peak temperatures used commonly elsewhere because your clay varieties have their own unique chemistry, not found elsewhere. Lastly, Dr. Sutharsan specifically recommended the red clay not be used in ceramics due to its high iron, alumina, and particle sizes. I believe you can use it, but you will have to develop your own customized formulation criteria to overcome its properties. Other articles reported spikes in the iron content upwards of 8%, which I suspect is what has happened here.

Tom

 

Tom,

    Thank you so much for your detailed study of the issue and I really appreciate your effort and time., and yes I started adding 1% of sodium bentonite to our Terracotta clay body and it seems like it has solved a issue a bit but I m finding the other issues like the clay splitting when it comes out from the pugmill, I dont know whether its related to our crack issue, pls find the attached pic of the clay., and its really difficult for me to add sodium bentonite more than 2% because it has the swelling property which in one side its good interms of slower drying and difficult interms of processing with the regular clay its not mixing well with the clay and the below two pictures are terracotta clay with 1% Sodium bentonite into it, do you think that the splitting isssue is because of adding the bentonite. ?

[Gokul]

On 5/6/2018 at 1:03 PM, curt said:

Edit:  To my eye the picture below, from Hamer and Hamer Dictionary of Ceramics, under pretty much sums up your cracking problem.   Bisquing only to Cone 08 (954 Celsius) seems very low temperature to me, and consistent with what is said below may be part of your problem.  Not saying the things mentioned by others above are not also part of the problem, but this to me is the main issue.  Solution is bisque fire higher (1000 or 1030 Celsius, and cool slower from the bisque, and don’t stack pots inside of each other in the bisque.

also, is the top picture you provided terra cotta or a white stoneware body?  It seems to have the same cracking problem, and if it is stoneware this would suggest your problem is not the clay body.

 

Curt,

   Thanks for your reply, and the top picture is our regular terracotta which is been throwing on the wheel. 

[glazenerd]

Gokul:

I would need more details about your processing and formulation before I could make determinations. I can see things in your  latest pictures that concern me. The first being color streaks: which suggests that materials are not being blended thoroughly. Are you dry blending all materials before you mix?  I can see surface blisters, which I I'll assume is air pockets? I know de- airing equipment is not readily used in your area. 

From reading Dr. Sutharsan,s study, the laterite is mined from under water, and used pretty much as is in the brick and roof tile industry. Given the hematite levels, along with calcium, sodium, and fluorite content; it vitrifies without any additional fluxes. So I will also assume you are just adding 1% bentonite to the powdered form, without any other additions? As I also know your red clay does not appear red until it oxidizes because of the hematite.

One of the big issues is the sesquioxides, which make up nearly a third of this natural clay. To give readers an idea of how these react in a clay body: imagine blending alumina and grog together, then trying to make them bind together in a malleable ball. It would be the same as throwing 30% of beach sand in a clay body and expecting it to hold its shape. Western clay formulation practices are not going to work in this case: you are dealing with a very unique clay variety, that has its own unique chemistry.

i know ball clays are available to you, and I would personally be exploring that avenue. I would start with an 80/20 blend of red clay and ball clay, and skip the bentonite all together. I realize 20% ball clay is on the high side, but you are dealing with a silty/ course clay. However, the particle size, CEC value, and WOPL of the ball clay you select , will ultimately determine the recipe %. If you do not have the capacity to dry blend before mixing, this will be problem as well.

Not much more I can add, I am sorta shooting in the dark without knowing the specifics. 

Tom

 

[Gokul]

1 hour ago, glazenerd said:

Gokul:

I would need more details about your processing and formulation before I could make determinations. I can see things in your  latest pictures that concern me. The first being color streaks: which suggests that materials are not being blended thoroughly. Are you dry blending all materials before you mix?  I can see surface blisters, which I I'll assume is air pockets? I know de- airing equipment is not readily used in your area. 

From reading Dr. Sutharsan,s study, the laterite is mined from under water, and used pretty much as is in the brick and roof tile industry. Given the hematite levels, along with calcium, sodium, and fluorite content; it vitrifies without any additional fluxes. So I will also assume you are just adding 1% bentonite to the powdered form, without any other additions? As I also know your red clay does not appear red until it oxidizes because of the hematite.

One of the big issues is the sesquioxides, which make up nearly a third of this natural clay. To give readers an idea of how these react in a clay body: imagine blending alumina and grog together, then trying to make them bind together in a malleable ball. It would be the same as throwing 30% of beach sand in a clay body and expecting it to hold its shape. Western clay formulation practices are not going to work in this case: you are dealing with a very unique clay variety, that has its own unique chemistry.

i know ball clays are available to you, and I would personally be exploring that avenue. I would start with an 80/20 blend of red clay and ball clay, and skip the bentonite all together. I realize 20% ball clay is on the high side, but you are dealing with a silty/ course clay. However, the particle size, CEC value, and WOPL of the ball clay you select , will ultimately determine the recipe %. If you do not have the capacity to dry blend before mixing, this will be problem as well.

Not much more I can add, I am sorta shooting in the dark without knowing the specifics. 

Tom

 

Tom,

   Pls find the attached picture of the clay which we are blending and using., These are the two red clays which we are getting from our supplier No.1 Red one  seems like the laterite or red soil after I read the article of Dr. sudharsans article abstract and the No.2 seems like more of an alluvial soil I guess so, and we used to get the No.2 most of the time becasue it used to be very good interms of plasticity and there wont be any cracking as well but the No.2 Mines clay is not easily available I can get the material only at the time of summer because the mines will be closed at the time of Mansoon.

So we cant and we dont want to rely on the brown one itself then we got the red one (No.1) which is very fine interms of particle size distribution and it has less plasticity ., so right now what we are doing is we get a stock of No.2 clay as and when its available and we mix 50% of No.1 and 50% of No.2 clay and we do our processing and make it into a clay. 

But still we have the cracks issue so we started adding 1% Sodium Bentonite with the above mixture mentioned and we make it as clay and pug it using the pugmill (Pic Attached) its a deairing pugmill but it dosent have any vacccum attached it and its a good homogeniser.

Pls find the attached pic of the process flow chart of our clay making process.

And the WOPT of our clay is 23 %.

Dry Shrinkage - 6 % 

Fired Shrinkage - 8% @(cone 08)

LOI - 15% @(cone 08)

Porosity - 20% @(cone 08)

Pls find the wet sieve analysis result below of the two clays

Sieve No	No.1	No.2
40	0.1	0.2
60	1	0.7
80	2.2	2.5
100	3.8	4.9
120	3.2	3.9
140	7	9.8
170	8	11.1
200	11.2	30.4
230	12.9	15
270	8.7	2.1
300	15.2	15.7
PAN (300+)	26.7	3.7
Total	100	100

[glazenerd]

Gokul:

Did some chemical research on laterite; and am coming to the conclusion this problem requires a chemistry solution.

Laterite is formed as silt in active river beds, under highly acidic conditions. The acidity strips away negative ions such as calcium, sodium and potassium. As they dig through the layers, the levels of these ions are depleted: which explains the zero calcium analysis. The next problem being iron, magnesium, and nickel are found in much higher levels, and usually as salts. The final issue is that hematite has anti-ferromagnetic properties, including on a molecular level. Which in this case has an effect on plasticity, seeing as though plasticity is based on negative ionic repulsion of adjoining clay particles. Hematite directly interferes with that molecular activity. Laterite  is so high in alumina, they use to process pure aluminum from it. So in pottery term; your red clay has every property that hinders ionic exchange: which translates to every possible anti- plastic property you can encounter. Oh joy!!!!! 

If you have a PH meter available, I need to know the PH of your pugged clay. That will tell me what minerals to add to overcome the lack of ionic ionic exchange. I think that is the root cause here: lack of ion exchange. No exchange = no plasticity. Secondly, I need a chemical analysis of your ball clay. 

Your process is fine, the fact that you filter press and still have plasticity issues astounds me actually. At this point, I think you need 0.50 to 1.00% barium carbonate to deal with the scumming and break down the soluble salts to start. After that, probably 1-4% calcium carb to replace depleted negative ions and to counter the acidity of this clay. You will have to a do a series of 1000 gram test batches to figure this out before you commit to production. I do believe the positive ionic composition of this clay is causing the shredding seen in your pics. Very odd and curious clay issue; but challenges intrigue me. Nothing like a good puzzle!!

Tom

 

[glazenerd]

Gokul:

Spent the last week reading up on your red clay-laterite. I find the chemistry of this clay very intriquing: unlike typical Western clays that have defined reactions.

The most compelling insights came from a study done by The British Ceramic Society, which used the term "cementing" to describe the properties. This term was used to describe its properties of wanting to compact into a dense ball. The 5-14% iron content! along with 30-35% alumina , coupled with the acidic nature; is creating its natural resistance to the usual plasticity fixes used in formulation. In plainer terms, by nature it wants to flocculate ( heavily).

The solution may include using a high alkaline negative ionic additive. Calcium is an important part of this puzzle because of it cation exchange rate. Sodium has higher cation exchange values, but calcium actually exchanges ions at a rate 200 times higher than sodium. Which I why calcium bentonite works better thn sodium bentonite.

https://trid.trb.org/view/884695.  This short abstract defines some of your issues.

If you look at India's natural resource maps from the Dept. Of Mines: you will see a very small region where high alkaline clay deposits are found. India has very little deposits, as compared to the West. You need to find a supplier who mines in this area. High alkaline ball clay will be lower in alumina, lower in iron, and richer in calcium: all of which you need. Your ball and red clay are approaching kaolin in alumina content: an oddity of itself. 

You are dealing with high iron, high alumina, acidic, and high carbon content clays. Having any one of these four require special formulation practices: but you have all four. A very tough problem indeed.

tom

[D.M.Ernst]

Glazenerd you boggle my mind.  Thank you for teaching us so much about glazes and now clay.  I am most impressed.  D

[glazenerd]

DM

thank you for your kind words. I am in good company, this forum has a wealth of people with differing expertise. 

T

[Tyler Miller]

On 4/12/2018 at 4:18 PM, Mark C. said:

The clay looks short as we say and needs to be more plastic. It looks like to much water is being used too throw it. This will also make it crack more. I would add more materials to make it more plastic and  age it and use less water when working with it.

This is what’s up.  I’m willing to bet that the wopl is a higher than 23%.  The clays you’re using are pretty variable in their composition (likely the most variable you can use).   It would jive with what Callie, Curt, and Yappystudent are saying.

It would also explain why sodium bentonite helped a little, but not completely. 

Working to amend the clay may not be as economically viable as finding a different supplier or stock-piling your more reliable clay for monsoon season.

[Min]

1 hour ago, Tyler Miller said:

 

Working to amend the clay may not be as economically viable as finding a different supplier or stock-piling your more reliable clay for monsoon season.

Exactly. 

[glazenerd]

Gokul:

Here is a link to some chemical analysis of your laterite clay. The only one I have been able to find from a reputable source: after two weeks of looking: http://sciencejournal.in/data/documents/SCIENCE-VOL-1-2-3.pdf. 

You will find a table on page three. Glad you are the production manager at this clay company, and not me. From what I have read thus far, India has some very unusual clay composition: very much different than Western clays. Combined total of alumina and iron is over 60%- wow!  No wonder you cannot fire over 900C.  If the duty taxes and tariffs were not so high, I would suggest importing a fix: but that would be very costly.  Still digging around for local resources, which seem to be very limited.

[glazenerd]

Gokul:

i have been looking for this link for some time, and came across it today while reading some notes. 

http://lawr.ucdavis.edu/classes/ssc219/biogeo/exchca.htm#clay

some definitions before you read

1:1 describes a clay particle: in particular kaolin. On a microscopic level kaolin looks like a saltine cracker: a single layer with alumina one one side and silicon on the other. This structure is the reason kaolin is mostly clean: no iron, magnesium, sulfides- etc. but it is also the reason kaolin is mostly non- plastic: it has no inner layers in which to hold water. For this same reason, it dries much more rapidly.

2:1 describes a clay particle with two outer layers of silicone; with one inner layer of alumina. On a microscopic level it looks like a sponge. Ball clay, bentonite, hectorite, and macaloid are all 2:1 clay particles. 2:1 particles do hold and absorb water: pending their structure and chemical composition: they can hold lots of water. Bentonite can hold up to 15 times it's weight in water. This absorption plays a role in plasticity, and also explains why stoneware takes longer to dry.

be sure to click and view the table for a CEC and AEC. Important section for you to understand.

cation exchange = cations are positive! and they are exchanged for negative anions. The more negative the charge, the greater the plasticity.

anion exchange:  anions are negative and are exchanged for cations resulting in a net positive particle charge. 

There are three points of interest to you: PH,  anion exchange, and hydroxols/ oxides. Most all of your clay is acidic (PH), most all of it is classified as " kaolinitic" meaning even your ball clays have kaolin like properties, and lastly: sesquioxides. Most all of your clay has higher levels. 

Pay close attention to the AEC levels associated with kaolin. Strong AEC levels combined with acidity creates the problem known as cementing. Cementing means clay particles are tightly bonded, creating a cementing effect. The shearing of your fresh pug shown above attests to the AEC of your clay. 

Simple clay plasticity rule: CEC = deflocculation = plasticity.  AEC = flocculation = colloidal cementation. both are PH dependent.

terra sig works because sodium silicate averages 11 PH ( high alkalinity). Drop just a few drops of sodium silicate into a bowl of slip and watch the film spread across the  surface. You are watching  the effects of a negative ionic charge spreading. (CEC)  vinegar works in throwing water because it repels the clay from your fingers. Vinegar ( acidic) works when joining pieces because treating the joints  causes flocs ( particle accumulation) to occur. iE: sticky clay. Bentonites work because they run between 9-11 PH.( in part) 

 Sharing some info while I have time.

tom

[Magnolia Mud Research]

Tom,

You should find these "texts" informative; the authors of both books discuss, and illustrate with examples, the physical chemistry of surface charges and the effects of those charges on solution chemistry.  

Title: The chemistry of soils
Author: Sposito, Garrison, 1939- author.
ISBN: 9780190630881
Publisher: Oxford University Press; 3 edition 
Edition: Third edition(November 21, 2016)

and

Physical Chemistry of Surfaces, 6th Edition
Arthur W. Adamson, Alice P. Gast
ISBN: 978-0-471-14873-9
Wiley-Interscience, Aug 1997

LT

[glazenerd]

LT

chapter 6: nucleation and crystal growth

i'm in!  TY.      Tom