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High Bridge Pottery

Matte To Gloss Glaze Chemisty With Al2O3

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So I thought I slightly understood glaze chemistry. Been having a look at a few of my tests in more depth.

 

Here is a 'glaze' that is going from this very stoney matte to glossy with only really adding Kaolin  :unsure:

The bottom is the left table, middle middle and up right.

 

Thinks Kaolin = refractory stuff, so it wouldn't make something go glossy.  All that really seems to be changing in the chemistry is the increasing Alumina, in non unity the silica hardly moves so I think that jump in unity value is because of the increase in alumina too.

gallery_23281_912_83685.jpg

 

gallery_23281_1027_104567.png

 

gallery_23281_1027_19513.png

 

All the flux going down, Al2O3 going up, SiO2 staying about the same. Less flux, more alumina = gloss. Very confusing.

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Kaolin supplies more than just alumina. Your feldspar supplies more alumina than your kaolin, typically. Check the back end of the program to see what each mineral supplies. I found it surprising.

If you look at your ratios of Al:Si:total flux, you can see the flux rising steadily in relation to the other two. Seen in that light, it makes total sense.

Hooray for molar weight!

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To add to Diesel's explanation, the quality of gloss vs matte in a glaze is a NOT a matter of melt in properly formulated glazes. It is a matter of glass vs crystal molecular structure. That being said, if you added enough kaolin, the glaze would devitrify, and the trend would support your hypothesis of adding refractory material causing a glaze to lose its gloss.

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Not sure I can see that light yet. There is a higher ratio of flux in the matte glaze compared to the gloss. 

 

If I was going to look at the chemistry I would think hey that one has the highest silica/alumina ratio and the most flux so it must be the glossy one of the three, but it is the opposite to that. I start with some silica and flux, taking out some flux, adding in kaolin and it is glossing up. Really interesting that the sweet spot seems to be a ratio of 7.5 for the best glass. Seems to be a little higher for cone 6.

 

It had always been in my head that a higher silica/alumina ratio the better the melt. I guess that is the rule but on both ends you stuff to much into the melt and it matts out. Interesting to look at the different surface quality between too much silica or too much alumina.

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Are you remembering to count your calcium and magnesium as flux? They are also melters at this range. Your main source of alumina isn't the kaolin, it's the feldspar. When you removed the feldspar in the name of taking out flux, it took out a piece of alumina, and when you added kaolin you added silica, bringing the ratio of the two of them closer together. However it also means there's a lot more flux compared to both silica and alumina.

 

That's how you can get a shiny glaze that dissolves really easily in vinegar.

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So I need to be doing total moles of silica+alumina / Flux ? Not flux / silica+alumina. 

 

It still doesn't make any sense. it looks like I am taking out flux and putting in alumina. The numbers of the fluxes go down and the rest up  :wacko: I still can't get my head round that the smaller the silica/alumina and bigger the flux the less flux there is overall.

 

Take the matte glaze, it has about 40 moles flux and 97 moles silicalumina. The glossy has about 30 moles of flux and 103 silicalumina. I thought 40 in relation to 97 is way more than 30 to 103.

 

I need to stop thinking about it, I am not sure what is right at all any more. Am I just doing the maths/thoughts completely wrong?

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:D ok. That is just Si/Al ratio. The Silica value has a decimal place.

 

So after thinking for a bit, the unity is the ratio of flux to everything else!! I don't even know why I was working it out. It is always 1 flux to X everything else. I think you confused me a little  :P but I am still amazed that less flux more alumina makes something melt. So amazed that I forgot how to think.

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I'm obviously misunderstanding either High-Bridge's data or Diesel's analysis (if not both).

So here is my arithmetic and analysis to be shot at.

 

glaze     table finish   flux     Si      Al        flux/(Al+Si)   [numbers from unity formula]

bottom  LH     matte     1.0   0.15   2.27   0.41

middle   ..       .....        1.0  0.27   2.59    0.35

top        RH    glossy    1.0   0.42  3.00    0.29

 

So as both the Si and Al increase relative to the total flux the glaze goes from matte to glossy

(for these 3 glazes).

 

This arithmetic is consistent with High-Bridge's statement:

There is a higher ratio of flux in the matte glaze compared to the gloss.

 

PS Table came out OK on preview, hope it stands up to distribution.

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Peter, the molar ratio of silica:alumina:flux is given on the screen shot is 14:8:1 for the matte and 7:2:1 for the shiny. For one molecule of flux there are 14 molecules of silica and 8of alumina in the matte. In the shiny glaze there are only 7 molecules of silica and 2 of alumina for each molecule of flux. So the flux is actually going up, not down, proportionately.

Because you can actually write a unity formula with silica, alumina or flux as the unity (or"1") molecule, this is where some of the confusion was coming from. It's standard to write it as Si:Al:flux, but you can change settings on some glaze calc software so it reads differently, as it suits your purposes.

 

Joel it helps to stop thinking of feldspar solely as flux. It is, but it also supplies more alumina than kaolin does. Likewise, kaolin is much more silica than alumina. Check out what each material supplies predominately in the back end of that program of yours, and it makes much more sense.

I'm sorry I confused you!

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Sorry Diesel but the 14.8 (fourteen point eight) is the silica value to the 1 alumina, tried to say that above. There is no flux ratio up there besides the unity formula which IS the ratio of silica/alumina to 1 flux  :D

 

The glaze is increasing in alumina and decreasing in flux towards the gloss.. Kaolin is 40% Alumina while my feldspar is 18% so removing the feldspar is not taking out more alumina that adding in kaolin gram for gram.

 

PeterH looks good to me.

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just looking at the silica : alumina ratio. first glaze at approx 15:1, second at approx 10:1 and last glaze at approx 7:1  Being that the flux levels didn’t change in any of the 3 glazes in theory the first glaze should be a very high gloss, second glaze a gloss and the third falls in the semi matte to matte range. any chance there was a user error somewhere and things got reversed? :unsure:


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Diesel, that's just the sort of misreading I thought I might be making.

 

However, looking at the top LH unity formula.

- The NxO add up to 1.00

- The Si:Al ratio is given as 14.8:1 that is 14 point 8 to 1 (rather than Si:Al:NxO as 14:8:1)

 

Regrettably,  I calculate Si/Al = 2.27/0.15 = 15.1  :(

... as Min got 15.1 as well something strange is going on.

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I'm not sure if this is cone 6 or 10, but if you plug them into unity formulas and you'll see that the calcium is off the charts for cone 6 and near the top for cone 10, and the magnesium is at the top for both. In the first tile the alumina and silica are so low that they are barely making it to the bottom end of limits. There's just not enough alumina and silica to take in all the calcium and magnesium and form a decent glass. Ever fire a fusion button of plain whiting or mag carb? Both come out dry white. In the second tile the alumina and silica are getting better and you start to see better glass forming. By the third tile the alumina and silica are well within limits, high enough that they can take in enough of the calcium and magnesium to form a nice glossy glass. It may still not be enough to form a durable glass, though, with the fluxes so high. And although the alumina is much higher in the third tile than in the first two, it's still not enough to cause mattness because the calcium is so very high.

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Guest JBaymore

Nothing beats Unity formulas for understanding this stuff.  Neil nails it.

 

best,

 

...............john

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Its a cone 9 firing :D

 

I get what you are saying Neil but the data just isn't saying that to me. I made myself a table with surface quality, ratio of silica+alumina to flux and Silica to Alumina.

 

If you look at the first picture, glaze 29 which is the matt glaze and say glaze 13 which has less silica+alumina for flux. Glaze 13 is a glass where 29 is matt. The difference I can see is the ratio of silica and alumina. Some are glossing with less than 2 silica+alumina to 1 flux. I know they are not technically a glaze but they are melting much more than others. I may have been a little generous with the surface definition of gloss but they are glossy, some of the outlier glossy could be classified as a half.

gallery_23281_1027_52064.png

 

The second ordering with Si:Al ratios seems to say that for a gloss need to be between 6 and 9. Many exceptions but thats a vague rule I can see.

 

gallery_23281_1027_62410.png

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Guest JBaymore

 

Thinks Kaolin = refractory stuff, so it wouldn't make something go glossy.  

 

Materials based thinking (as in >>feldspars are fluxes<<  >>>kaolin is a refractory<<<  >>>barium carbonate is a flux<<<) is where many of the problems arise.

 

Materials are suppliers of oxides in the FIRED FORM.

 

The OXIDES are the things that you are concerned with when you look at FIRED glazes (raw material sources affect the glaze slurry, and to a degree the WAY that the melt forms).

 

Carbonates are not fluxes on silica.  Ditto hydroxides.  Ditto chlorides.  Ditto other chemical forms.... other than the oxides.  Formal definition:  "Ceramics is a technology based on the oxides of metals and metallic earths."

 

Kaolins supply oxides in the melt.  Some of the oxides it supplies take the function of fluxes.  Some take the action of glass formers.  Some take the function of ampoterics (which is an interesting beast that can share some characteristic effects of other classes of oxides on the glaze melt).  You have to look at the analysis of a particular kaolin to know what relationship the various oxides are supplied in. (Ditto for other raw materials sources.)

 

Then we get to an often overlooked area:  the body / glaze interface layer characteristics.  Important stuff.  Particularly at high fire like cone 10. 

 

So the THICKNESS OF APPLICATION of the glaze material over the underlying body can have a large impact on the melt. (As can the chemical and physical composition of the body.)   Because materials from the body get into the glaze layer, and materials from the glaze eat into the clay body.  Something as simple as the thickness of the layer can take a glossy glaze and make it a matt... and a matt and make it more glossy.  (This is where a lot of "what went wrong" stuff shows up for people when they open kilns.)

 

The Curie grid system is very prone to problems with the latter.  Personally, I'm not a big fan of that system......other than it provides a "structure" for organizing testing work (which IS important).

 

When a glaze is oversupplied with fluxes relative to the glass formers (high ends of limits for a given fluxing oxide), adding something that supplies silica (like a clay... which materials based thinking says is a refractory) can make it go glossy.

 

Then there is a really important issue;  the accuracy of the materials analysis you use in the glaze calc program.  That old computer adage fits here:  "garbage in = garbage out".  You have to keep getting "typical analysis" sheets from YOUR supplier.... and updating them regularly.  Raw materials change.  Some suppliers have more pure materials than others. Even keeping up with this regularly... you have to remember that term "TYPICAL" analysis.  There always can be some variation in the stuff in the bag that you just used.... from the analysis that your supplier just sent you.

 

There is an "art" component to controlling a technology that is heavily based on stuff dug out of the earth.  This is why even industry uses TESTING to assure consistency.  BEFORE committing to huge production runs.

 

This is complicated stuff.  That is why god created ceramic engineers ;) .

 

best,

 

.......................john

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Guest JBaymore

Thanks, John. Were you lurking about in the shadows, waiting to see if someone figured it out? :-)

 

No Neil... just no time to do much writing on here.  This is a crazy fall for me.  Won't get better until after the 1st of the year.

 

I do Mod work... occasionally write mainly short items/comments.

 

best,

 

.......................john

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I guess the point I am trying to notice is the amphoteric property of Al2O3, in a silica rich or poor glaze chemistry it seems to help the melt. Then you go past and it starts to impart different characteristics.

 

In trying to notice I seem to have made a Fup.

 

I have been making a mistake in thinking the silica in the glaze doesn't change because the non unity hardly moves but the amount in relation to flux does! My ideas about only swapping out the flux and Al2O3 have been misdirected by a load of silica I seem to have forgotten about. Both directions on the tile increase silica 

:o DOH The total silica hardly changes but in relation to the flux I take out it does :D:D  <_<

Seems my understanding of non unity and unity was a little off.

 

Now I do have something I could try and test, if Al2O3 does flux a silica rich/poor glaze.

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