Brand new glazes, brand new issues

[Chloedabit]

Hello all! 

Our studio has been using Amaco glazes for a long long time <3 we just got a new class pack and I went through to make all the new test tiles. I waited and waited to unload the kiln, and to my astonishment - none of the tiles came out!! I thoroughly  mixed each glaze with a clean stick, used a brand new clean brush for each application, did a SOLID 3 coats of each. fired to the right cone (6) and have ensured the temperature was reached via use of cones in our kiln. Still, it seemed like each color flipped. The main color was the color it would usually “break” as. For example, oolong gloss comes out deep red with green in the “break” areas. Instead of green with deep Red where it breaks. Halp! I don’t know what else to try! Any suggestions are welcomed. Thanks you wonderful clay community!

[Mark C.]

wrong firing temps?? kiln fired cool? fired hot? application different? different clay body?Lots of ways to get this wrong.

Flipped ?? you mean like  states evidence?  or glaze is on bottom of tile?

[Benzine]

Are the glazes correct, for the type of firing you are doing, in regards to it being a reduction or oxidation firing.

I would imagine the results would be quite different, if you used a reduction glaze, in oxidation and vice-versa.  I know the Raku glazes I've used are quite different, when fired in oxidation, as opposed to putting them in the reduction bins.

[neilestrick]

What color clay? The tiles you see in the catalogs are on white clay unless otherwise noted. They also use a certain firing schedule, a certain glaze thickness, etc, and they pick the best tiles from all their samples. It could be that they're going to look different on your clay, fired in your kiln. Run another batch and see what happens.

[Gabby]

I use those Potters Choice shino glazes. What I have found is that you have to apply the three coats very heavily if you don't want them to be almost all the rust color.

Also, these definitely need to be tested for whether they are food safe. I have made several pieces that were not food safe, though I had followed directions.

I have used them on porcelain and on red stoneware.

[neilestrick]

38 minutes ago, Gabby said:

I use those Potters Choice shino glazes. What I have found is that you have to apply the three coats very heavily if you don't want them to be almost all the rust color.

Also, these definitely need to be tested for whether they are food safe. I have made several pieces that were not food safe, though I had followed directions.

I have used them on porcelain and on red stoneware.

How did you determine that they were not food safe?

[Gabby]

1 minute ago, neilestrick said:

How did you determine that they were not food safe?

I understood that if a food leaves a stain that will not come out, one should assume the dish is not food safe.

So I let a tablespoon of oil sit on it for an hour, then washed the bowl and let it dry. 

 

[Joseph Fireborn]

1 hour ago, Gabby said:

I understood that if a food leaves a stain that will not come out, one should assume the dish is not food safe.

So I let a tablespoon of oil sit on it for an hour, then washed the bowl and let it dry. 

 

There is a difference between food safe and durable. What you describe can be both, if the glaze contains high amounts of potentially "bad" ingredients and the surface is damaged by some oil for an hour then yes, it might be unsafe, but in general that more describes something that is non-durable. If the glaze is non-durable it doesn't instantly make it not safe, depends on whats in the glaze. However, you should still refrain from using this glaze on pots that will have food in general because it will resemble bad quality work.

[Min]

2 hours ago, Gabby said:

I understood that if a food leaves a stain that will not come out, one should assume the dish is not food safe.

So I let a tablespoon of oil sit on it for an hour, then washed the bowl and let it dry. 

Are you sure your clay and the Potters Choice Shino glaze was fired to maturity? You're not using a wide firing clay are you? Like one that goes from 6 - 10? I don’t use these glazes but a quick look up of information shows them listed as dinnerware safe. Underfired glaze and / or clay would cause food staining. Even if you are only using these glazes for non-functional work I don't think the glaze should stain.

BTW, tests to see if your glaze is definitely leaching would be one for acid, one for base.

Acid: Half fill a glazed pot with vinegar, cover with plastic film to stop it evaporating, and leave it for several days. Empty the pot, rinse and dry thoroughly. Look for change in colour or finish. Can also use a thick crosscut slice of lemon instead of the vinegar. Lemon slice on a flat test piece, cover with plastic film and leave it sit for a few days in the fridge, rinse and dry.

Base: Have 2 test pieces glazed and fired the same way. 1 litre of water plus 50 grams of soda ash in a saucepan, simmer 1 glazed test piece for 6 hours. Rinse and dry and check for change in glaze colour or finish to untested piece.

Can also run a piece through the dishwasher for several months and do a visual comparison to check for changes. (I prefer this to the soda ash boil test)

These tests tell you if a glaze is leaching. They don’t tell you if the glaze is “safe”. If there is nothing in the glaze to cause harm then not much to worry about, problem with commercial glazes is you don’t know what is in them. (Lab testing is the only sure fire way to know if a glaze is leaching.)

[Gabby]

Thank you, Joseph and Min.

I use clays that are for Cone 5-6, not wide ranging.

I don't fire my own. I take it to a place with a lot of experience that does the firing for a small charge. 

I think I may not have applied the glaze thickly enough, or that there were places it wasn't thick enough, as I brush the glaze on.

I am sure Amaco's assessment is correct, but I am a source of human error.