Silica In Glaze And Body To Ensure Good Fit?

[PottaFella]

Am I right in thinking that it is the percentage of silica (silica dioxide?) in a glaze and clay body that determines the fit? If both glaze and body have same percentage of silica then the fit would be perfect with no crazing or shivering?

 

Probably isn't that simple?

[glazenerd]

Material mass.

             L = 1    dL                 

                   L    dT

 

The (L) for a solid (clay), and a liquid (glazes) are not the same. You see the glaze as a solid after cooling: but during the phase change (liquid to solid) is when COE is measured. Clay has mass, so the phase transition is entirely different.   Kinetic energy. rate changes, phase transitions, Hess law ... etc, etc. etc. .blah, blah, blah..

 

  Nerd....   I need to make a few fun posts: I am starting to bore myself.

[neilestrick]

What Nerd said. Not that I understand what he said.

[neilestrick]

 

 

  Nerd....   I need to make a few fun posts: I am starting to bore myself.

 

Cat videos are always good.

[glazenerd]

 

What can I say Neil?

 

Nerd

[PottaFella]

Started to take note that the tech info sheet by Valentines Clays give the breakdown of a clay's elements and also shrinkage rate.

 

The Michael Bailey Cone 6 glazes book gives the 'COE' of most of his recipes and clues as to how to use them.

 

It seems a glaze with COE of 5 or 6(?) I think gives best chance of it being a good fit on most clays?

 

Porcelain is most difficult to deal with because it shrinks so much?

[PeterH]

Am I right in thinking that it is the percentage of silica (silica dioxide?) in a glaze and clay body that determines the fit? If both glaze and body have same percentage of silica then the fit would be perfect with no crazing or shivering?

 

Probably isn't that simple?

 

I'm afraid not. Indeed counter-intuitive even crystalline silicon dioxide (quartz) and fused 

silicon dioxide (a glass) have quite different thermal expansion properties. So its not

just the amount of SiO2 but also its chemical/crystallographic form.

 

To emphasise the point, fused silica has a low thermal expansion and is the gold-standard

for low-expansion heat-resistant laboratory glassware. On the other hand, quartz:

 

https://digitalfire.com/4sight/glossary/glossary_cristobalite_inversion.html

Quartz has one physical property that is a bane in ceramics: A sudden 0.5% increase in volume change as it is heated up through a narrow 50C window of temperatures centering around 550C (it contracts by the same amount as it is cooled through this temperature). Cristobalite, not to be outdone, does the same thing but at a much lower temperature and more suddenly (0.8% change in 30 degrees C centering around 200C). While many books and references state that cristobalite inversion happens at 220C, an examination of a graph of its expansion vs. temperature (see link below or google images for the term "quartz vs cristobalite inversion") shows that it is alot more complicated than that. Cristobalite begins expanding suddenly right from the start, the rate of increase accelerates to a near vertical line, then drops off to a much slow rate of increase.

 

Basically a lot of the SiO2 in a body is crystalline silica, and most SiO2 in a glaze is part of

a glassy matrix.  So its properties are quite different in the two situations.

 

I'm afraid its back to the usual sources of information:

https://digitalfire.com/4sight/glossary/glossary_coe_co-efficient_of_thermal_expansion.html

https://digitalfire.com/4sight/troubleshooting/ceramic_troubleshooting_glaze_crazing.html

[PottaFella]

There's a lot to take in here.... if I have got it right?

 

 

1. Need a clay body that vitrifies at my firing temperature.

 

2. Know the expansion/contraction of the clay?

 

3. Know my chosen glaze recipe is formulated correctly for the temperature and melts right and is going to be durable?

 

4. Know how a glaze recipe will act on my chosen clay (COE)?

 

5. Test it to be sure with some sort of extreme temperature/freezing/boiling test and durability test? 

[PeterH]

Preamble

- Firstly, I'm not the right person to ask.

- Secondly, yes -- in theory -- its as complicated as that.

- Thirdly, I don't think anybody normally does it that way.

 

Direct  top-of-the-head opinions on your points

 

1. Need a clay body that vitrifies at my firing temperature.

If you want your work to be really functional, otherwise you're relying on the glaze for waterproofing.

AFAIK you cannot do this with e/w as normally fired.

 

2. Know the expansion/contraction of the clay?

You wish. AFAIK it even depends on how you've fired it.

 

3. Know my chosen glaze recipe is formulated correctly for the temperature and melts right and is going to be durable?

Ideally. The glaze programs can help (assuming that its a glassy gaze). Things like under-firing a clear glaze to get a matt

are not a good idea. Durability has  received a lot more attention after Mastering Cone 6 Glazes by John Hesselberth and

Ron Roy came out.

 

4. Know how a glaze recipe will act on my chosen clay (COE)?

Some of the glaze programs can give you a estimate of the COE (assuming that it is a well-fired glassy glaze). AFAIK

the general idea it to learn what the estimated-COE is for glazes that do successfully fit your body (as you fire it) and

aim to formulate new glazes to something like that estimated-COE.

In practice I suspect that there is a lot of grabbing recipes from somewhere, finding ones that they nearly fit, and line-

blending to a good fit. [Good pragmatics are better than reliance on inadequate theory.]

Remember to rub-in indian ink or something when looking for crazing it shows things up wonderfully, even fresh cracks.

 

5. Test it to be sure with some sort of extreme temperature/freezing/boiling test and durability test?

If you're selling to the public it sounds like good insurance. For the hobbyist its still sounds like good practice, especially 

if you are using a porous body.

 

[Min]

Good on you Oly for asking the questions.

 

Just adding a bit to PeterH’s excellent post.

 

If I was starting from the beginning at sorting this all out for functional non leaking pots I would do something along these lines,

 

- test the body with no glaze for leaking/seeping, throwing and or hand building qualities etc

- in your case I really would take some time to do this properly as you have such a limited choice of midrange clay to choose from

- make up a series of glazes with increasing COE figures. There is a series of them in the Mastering Cone 6 Glazes book. I don’t think their low expansion one is low      enough though, I would add another really low COE one. (I found the Bailey ^6 book glazes tended to be higher in COE than a lot of recipes on this side of the pond.)

- run dunting/shivering tests plus crazing tests with the above glazes. Some will craze, some might shiver but hopefully there will be one that fits.

- start working with that one glaze. Really get to know it, add opacifiers, colourants, variegators etc to see what it does.

 

- if you are not already doing it then glaze software really helps cut down on testing glossy glazes that won’t fit your clay. The software won't rule a glaze in but helps weed some out. COE figures don’t work with matte glazes but you can still use them to see what is in your formula which is helpful.

[Bob Coyle]

Oly

 

No single component determines COE. it is a property of all of the bases present. If you know the COE of the clay body ( commercial bodies usually have this info) then all you have to do (theoretically) is match the COE of the glaze with the clay. There are many software programs that help you do that. From my experience they may not work as well as expected. They are good for a start, but they depend on a kind of a linear additive approach, which is not the way glaze works.  Adding or taking away SiO2 might get you to a better fit, but could alter the cone you need to fire to. The ramp you fire also effects  the final crystalline configuration of the glaze.  I have made glazes that calculated out with a close COE to the clay body, and well within major limits formulas and I still got some crazing.

[preeta]

wait a minute. so i should not trust what the manufacturer says about vitrification. i should test it also just to make sure? i can see testing the clay you mined and mixed. but commercial clay too? i am talking about commercial clay that is ^5/^6 range, not ^4-^10 range. 

 

what about flux? can we talk about silica without talking about fluxes. 

 

so shrinkage rate of the clay matters? meaning i have two ^6 clay bodies. one has a shrinkage rate of 10.5% and the other has a shrinkage rate of 13%. a ^6 glaze could behave differently on both bodies?   

[glazenerd]

Preeta:

 

If you are doing functional ware; then testing "everything" becomes a work standard. If you are not, far less concerns. Testing clay then becomes important if you are having crazing issues, weeping, or other problems that need resolved.

 

Flux additions show up on our glaze calculators as "molarity". The application however is concentration: or PPM (parts per million)/ Total alkali/total flux is indicating a ratio: how much flux is present to melt the silica/alumina content. There has to be a certain molarity level to melt all the silica and alumina: which changes according to the amount present, and the temperature. Cone 1 needs more, cone 6 needs less, and cone 10 needs even less: because increasing the heat lessens the amounts required. Silica melts without flux at 3150F, so we have to use flux to lower that temp.

 

Shrinkage matters if you are using more than one clay to make a piece, or if you are making a piece that requires you to hit a predetermined size after firing. Lids are a primary example of knowing how much something will shrink. Shrinkage does not really effect glaze fit very much: because at peak temps: the clay has done all the shrinking it is going to do: so the clay and glaze cool together. It is upon cooling that COE differentials between the clay and glaze become painfully obvious.

 

Tom

[Guest JBaymore]

preeta,

 

COE and what we typically call "shrinkage rate" are two different aspects of the materials.  They are not totally related to each other.

 

"Shrinkage rate" as potters use it basically means the permanent size change that happens as certain things happen to the clay body.  Some shrinkage happens as physical water of formation leaves the wet clay form.  This is usually called "wet to dry" shrinkage.   It is permanent (as long as you don't re-wet the unfired clay  ).  Then there is firing shrinkage as chemical water is driven off the clay crystals and the body starts to vitrify to one extent or another.  This is typically called "firing shrinkage".  It is permanent too.

 

This shrinkage can affect how a raw slip coating stays on the body as it shrinks.  This shrinkage can affect how, in the early stages of a firing, a raw not-yet-really-melting glaze layer might stay on the surface of the form.

 

Coefficient Of Thermal Expansion, or as I prefer to describe it to students as the REVERSIBLE Coefficient of Thermal Expansion, is related to a different situation for potters.  reversible means that as thngs heat up they expand and as they cool down they contract.  Important concept.  COE is dealing with size changes that are NOT permanent.  They are temperature dependent.

 

It becomes important when you have a FIRED clay body, and a chilled, or set molten glass layer on top of that body.  The idea of "set" when it comes to glaze means that the glass is now at a temperature below the state at which it is still fluid and able to flow to any degree.  The idea of "frozen water" is often used for this idea but that is technically incorrect.... because water is a liquid but ICE is a solid... with a regular crystalline arrangement of the water molecules. 

 

Remember as a side point that glass is not a solid even though our human senses tend to perceive it as such.  It is an amorphous association of the molecules that make it up.  Although a dated and slightly inaccurate expression.... glass is often called a "super-cooled liquid".

 

Once "set"..... like most materials, glass expands upon heating and contracts again upon cooling.  SO does fired clay.  The COE deals with THIS shrinkage aspect.  As the clay body and glass cool from the firing, you want them to shrink from THIS aspect (not related to the overall total shrinkage described above) at the same rate and amount.

 

If the glaze shrinks in this aspect more than the clay body... it becomes in tension.  Glasses are [pretty weak in tension.  SO the glass fractures to relieve the tension.  This is crazing.

 

If the glaze shrinks LESS than the clay body, this puts the glaze in compression.  Glass is quite strong in compression.  SO the glaze layer is now larger than the clay body under it.  This can cause stress relief by having the glass flake off the edges of the clay body.... called shivering.  In big cases... this can cause the clay body to rupture if the glass is stronger than the clay wall cross section.  This is one form of dunting.

 

A really bad combination for work is a piece with a glaze in compression on the inside, a glaze in tension on the outside, and a thin wall or weak clay body.  Explodo-pots.

 

Hope this description helps.

 

best,

 

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

[Bob Coyle]

Not much more needed from what John and glaze nerd said... except YES!  you really need to pre test every change in clay or glaze before doing a full batch.  Fire at as close as you can come to the conditions you will fire the batch. Firing cycle can also make a big difference in the final appearance of a glaze. many of us here have learned the hard way that even going from batch to batch of a commercial clay or glaze can produce problems, and this is especially true if you mix your own glazes and change raw materials.

[preeta]

   i will once again repeat my gratitude for the time you take to explain everything. right now i am living in the world of unity formulas, COE and the clay threads here.     its making me wake up at night with questions which makes me go online to find out and then i look outside and the night has passed. 

 

it is from the clay thread that i learnt that glass is a liquid but clay has mass even though it melts too. 

 

john i was concerned about the firing shrinkage (not the COE) because the couple of types of clay i use has varied 'fired shrinkage' rates. 

 

so Tom i am glad to hear that kind of shrinkage does not matter much.

 

reading Tom's clay thread it does not surprise me that i would have to test all clay. and i remember johns biggest fail (if i remember right) which reminds me to always test.

 

its been an eye opener how the process of milling affects how the claybody and glaze behaves. wow.

 

back to reading!!! 

[glazenerd]

Preeta:

 

Let me use something everybody is familiar with: food!

 

Glaze is like jello: you mix a granular material (glaze recipe) into hot water (flux), until everything is dissolved (peak temp), and as it cools it sets into a uniform gel.(amorphous glass)

 

Clay is like homemade cookies: flour (clay), butter (flux), sugar (silica), and flavors (alumina). As it heats, the butter melts first, which in turn liquefies the sugar, and pulls the flour into the melt, and the flavorings are disbursed. As it cools, the butter (flux), sugar (silica) set hard, and it becomes firm when cooled. Add chocolate chips to it: and it becomes stoneware.

 

Nerd

[preeta]

Nerd thanks to all of your guys explanations for the fast few months, i get the basics. 

 

i get the columns the Rs and Os

 

its moving beyond the point that is driving me batty. i am not complaining. i am loving it. its my kinda thing. but i do better in a classroom setting as i can ask my questions (instant gratification) as well as sometimes the proff. connects things that makes sense. on my own its tough esp. since i just have beginning college level science background. 

[High Bridge Pottery]

Where can I find the jello coated cookies?

[Chilly]

Where can I find the jello coated cookies?

 

 

I think they call them Jaffa cakes