Applying a very thin layer of clear glaze over painted porcelain

[thiamant]

Hi again!

I tried using the "famous" clear glaze G1947U from digitalfire over painted porcelain (with cobalt), to a specific gravity of 145. The result is completely clouded, with some microbubbles inside, the color isnt exactly what I was expecting (like beige white instead of clear white).

My thoughts:

- Thick application, maybe reduce SP GR to 135 or less?

- Try using a deflocculant?

- I run this on slow cooling cycle (I had other tests in the kiln which required this), maybe this is causing the bubbles inside the glaze? Are these bubbles or microcrystals?

 

[Bill Kielb]

Thin but fully covered for the clearest clear, regardless of fame is usually the rule. First it looks fairly thick. Slow cool will allow time for crystals to grow so maybe not ideal for a slow cool as anything that interferes with the light will make the glaze look more and more cloudy so to speak. Best I have read in some time is Sue McCloud: Getting clarity with clear glazes : https://suemcleodceramics.com/getting-clarity-with-clear-glazes/.  And ………… it’s free!

[Pres]

Bottom edge of glazed area indicates movement and thickness. Have you tried spraying the glaze on, and as @Bill Kielbsays crystallization is apparent nullifying the transparency, so a faster firing could help solve part of the problem.

 

best,

Pres

[neilestrick]

I've used a couple of the Digitalfire clear glazes, and found that I needed to flux them a bit more to increase clarity. But first apply it thinner, and make sure you're firing to a solid cone 10.

[Mark C.]

I have posted my clear glaze which is over cobalt drawinngs on plates and covered Jars-Its a cone 10 clear-applies very thin and fired hot.

You can find it in the search section and it was about two years ago .I would use another clear as that base looks to opaque

I can post that formula agin if you want it?I will be by my glaze book on Saturday .

[Callie Beller Diesel]

It should be noted that on the Plainsman website, G1947U is listed as a transparent, not a clear. It’s a cone 10 base glaze that isn’t intended to be used as-is, but as a starting point to get other things.

If you read down the page for the glaze notes, Tony makes the suggestion that frit could be substituted for the feldspar if you wanted to increase flow, or you could switch the EPK for grolleg to get better clear glaze results. https://digitalfire.com/recipe/g1947u

[thiamant]

I fired a couple of test tiles with different SG (135, 140, 145, 150) using G1947U. 

@140SG provided the best result:

It's not bad... But I still can see a bit of a yellowish tint, maybe because I should switch to a more pure source of kaolin? 

Also, there are a bit more microbubbles than what I would expect. Replacing potash feldspar with nephsy or sodium feldspar is going to help? (adjusting properly)

(I checked Sue's clear and people seem to like it a lot, but most bodies that I use are cone 10 )

[Bill Kielb]

3 hours ago, thiamant said:

(I checked Sue's clear and people seem to like it a lot, but most bodies that I use are cone 10 )

The intent in Sues  article was to provide an easy method for you to find a way to make your clear as good as it can be. Not necessarily use her clear.

[thiamant]

28 minutes ago, Bill Kielb said:

The intent in Sues  article was to provide an easy method for you to find a way to make your clear as good as it can be. Not necessarily use her clear.

Sure, even it might look like it's not the case, I have read the whole article 

I guess I could do different SiO2:Al2O3 tests, but this yellowish tint which is similar to my kaolin raw powder color is making me suspect that the kaolin is just not the best. 

[Bill Kielb]

5 minutes ago, thiamant said:

this yellowish tint which is similar to my kaolin raw powder color is making me suspect that the kaolin is just not the best. 

No worries, when you settle on final materials, you still have a way to possibly make IT as clear as it can be. (Less bubbles, etc….

[thiamant]

7 minutes ago, Bill Kielb said:

No worries, when you settle on final materials, you still have a way to possibly make IT as clear as it can be. (Less bubbles, etc….

You mean by experimenting with different chemistries and the stull map?

[Bill Kielb]

48 minutes ago, thiamant said:

You mean by experimenting with different chemistries and the stull map?

Sort of but not so complicated. When you are satisfied with your chems and recipe then  you have the ability to tweak to clearest by just adjusting the silica and alumina.

[Min]

5 hours ago, thiamant said:

I guess I could do different SiO2:Al2O3 tests, but this yellowish tint which is similar to my kaolin raw powder color is making me suspect that the kaolin is just not the best. 

Do you have an analysis for the kaolin you have? It can be quite interesting to fire material samples up to glaze temps to see their melt and colour. Just a 1/4 teaspoon or so in a depression in a slab of clay.

Simplest thing would be to try it with a cleaner kaolin. Can you get grolleg? If that still doesn't make it clean enough see if you can get a small amount of mahavir feldspar and swap the custer for that and add 2 more silica to balance it out. If you can get away without using bentonite with your grolleg do that, if not use veegum or bentone ma (aka macaloid) to suspend the glaze. Third option would be to use nepheline syenite and bump the silica up to 33.

 

[thiamant]

1 hour ago, Min said:

Do you have an analysis for the kaolin you have? It can be quite interesting to fire material samples up to glaze temps to see their melt and colour. Just a 1/4 teaspoon or so in a depression in a slab of clay.

Simplest thing would be to try it with a cleaner kaolin. Can you get grolleg? If that still doesn't make it clean enough see if you can get a small amount of mahavir feldspar and swap the custer for that and add 2 more silica to balance it out. If you can get away without using bentonite with your grolleg do that, if not use veegum or bentone ma (aka macaloid) to suspend the glaze. Third option would be to use nepheline syenite and bump the silica up to 33.

 

Definitely will check those. I have other sources of kaolin that I want to try (at least in the raw powder they seem more white). Not grolleg, I suppose. 

If I add say 0.2-0.5% TCP (bone ash) can I get a slightly blue tint maybe that could compensate for the yellowish colour? Can that mess the chemistry too much?

[Mark C.]

This glaze has worked well for 40 years at cone 10 the hotter the better. Apply thin and rub out any bubbles if dipping (I dip then blow on it so the surface is even)

Potash feldspar like custar -27

Ball Clay I use Kentucky ball clay -others will work-19.5

Whiting-19.5

Silica use 325 mesh-the finer the better-34

I use this over cobalt drawings or brushwork or cobalt pencil work

 

[Min]

1 hour ago, thiamant said:

If I add say 0.2-0.5% TCP (bone ash) can I get a slightly blue tint maybe that could compensate for the yellowish colour? Can that mess the chemistry too much?

Wouldn't be my first choice to counter yellow tinge but wouldn't hurt to try it. Don't forget that bone ash acts as both an opacifier and adds opalescence to a glaze but in such a minute amount it likely won't. Could also try a very tiny bit of blue stain, in the order of 0.01%  I would be looking at a different kaolin first. 

edit: on the molecular level there isn't a huge difference between Marks clear and this one, mostly the zinc, silica and alumina. Easy way to get rid of some of the bubbles would be to drop the zinc and increase the spar.

[thiamant]

59 minutes ago, Mark C. said:

This glaze has worked well for 40 years at cone 10 the hotter the better. Apply thin and rub out any bubbles if dipping (I dip then blow on it so the surface is even)

Potash feldspar like custar -27

Ball Clay I use Kentucky ball clay -others will work-19.5

Whiting-19.5

Silica use 325 mesh-the finer the better-34

I use this over cobalt drawings or brushwork or cobalt pencil work

 

Nice work! Gorgeous. 

Any reason to use ball clay instead of kaolin?

So after dipping you apply another coat by spraying?

[Mark C.]

The recipe calls for ball clay-so I use that not EPK

I do not spray another coat just dip a thin one

I made those pots but did not do the drawing a friend did. I glazed them. I use this glaze every year over drawings-usually cats.

It works why mess with it-I fire it in the hot spots of reduction kiln

[thiamant]

Based on G1947U I replaced custer with nephsy, removed zinc, increased R2O a little bit, and adjusted silica and alumina levels:

https://glazy.org/recipes/161722

I'll have to try this one with two different kaolins and see. If I had time I guess I could do a blend line or triaxial.. but I already have so much work  maybe in the future. 

[Min]

17 hours ago, thiamant said:

I'll have to try this one with two different kaolins and see. If I had time I guess I could do a blend line or triaxial.. but I already have so much work  maybe in the future. 

Since you are taking the time to mix up the test batch I'ld try and get a couple more results from it. I'ld mix up 200 grams of it with around 16% kaolin and dip a test tile. Add the rest of the kaolin to bring it up to the 19.5% and dip another test tile. Then add the 2.5% zinc and dip a third test tile. I'ld also place the test tiles on waster slabs since the glaze could run more. (which would probably distort the cobalt brushwork too) Try one dip over all the test tile and a double dip on a top corner.

[thiamant]

So finally I fired the tests and these are the results:

 

G1947U with grolleg kaolin substitution:

 

Same glaze sprayed:

 

NephSy clear with 15% grolleg (similar UMF than g1947U but using nephSy and no zinc)

Same glaze sprayed:

NephSy clear with 18 grolleg: (dipped)

NephSy clear with 18% grolleg and 2.5% zinc oxide (dipped, grayish tint):

 

From all these tests I think the best one is the NephSy clear 15% grolleg sprayed (thin), maybe a bit thinner would be perfect.  The differences in colour are very subtle but zinc tends to give a slight gray-ish tint. (Note that these images are very high resolution, so that every detail can be seen)

[Min]

For completeness of your testing you might want to do a stress test to check the fit of the glaze. The one you like the best with less clay and using nep sy is going to have a higher coefficient of expansion than the original so it might craze over time. Also, I'ld try to use a lower sg and dip if you plan on using this glaze a lot, much faster to dip than spray and for a clear you aren't looking for layering spray glaze effects. Link here if you need it on how to stress test for crazing. Thanks for posting your results.

[thiamant]

18 hours ago, Min said:

For completeness of your testing you might want to do a stress test to check the fit of the glaze. The one you like the best with less clay and using nep sy is going to have a higher coefficient of expansion than the original so it might craze over time. Also, I'ld try to use a lower sg and dip if you plan on using this glaze a lot, much faster to dip than spray and for a clear you aren't looking for layering spray glaze effects. Link here if you need it on how to stress test for crazing. Thanks for posting your results.

Sure, will do. It seems very easy. 

Spraying is nice for me because my studio is very small, I don't want to fill large buckets of glaze until I'm 100% sure that this glaze is ready to use in a larger scale. I've made that mistake before . Meanwhile spraying is very effective, I can prepare a small batch of glaze (100g) and use it for spraying a middle sized bowl or piece.

Will let you know about the stress test.

[thiamant]

After the stress tests (150ºC to ice water), all the nephsy clears had crazing, the one with the 2.5% zinc added had less crazing, but still crazed a bit. The G1947U with grolleg didn't have crazing. Sooo, all my dreams gone . I'm wondering however why the added zinc made the glaze craze less, since it has less alumina and silica molecules in it...

[Min]

2 hours ago, thiamant said:

Sooo, all my dreams gone 

This isn't how I would look at this. What you have found out is that over time those clears would likely craze using those ingredients in those proportions. Better to find that out now rather than after you made a bunch of work with it. Making a well fitting crystal clear glaze is one of the harder things to do and does take some work. 

When you add another ingredient (like zinc) the COE figures don't go in a linear fashion from one glaze to another without it. COE figures are good for comparing glazes when ingredients don't change but not necessarily between glazes using different materials. It's like adding a new family member to the home, the dynamics change.

Have a read through this thread on crazing with 1247U cone 10 clear and the solution the op came up with. Just a note, the fellow who was testing the low expansion clears in the thread I linked to eventually found out there was an issue with the manufacture of his claybody. Like all good dialogues there is some back and forth on methodology. ( I favour reducing high expansion fluxes for lower ones when the crazing is more than slight)

edit: how far apart are your craze lines?