Trying to raise the melting point of a commercial glaze

[Elmoclayman]

Hey all,

New to trying to adjust and mess with a store bought glaze.  I have a premixed Cone 10 turquoise glaze that I have used for over 15 years that I purchase from my local clay supplier, (Laguna Clay).  I usually fire in a reduction to the same temp and recently the color has been burning out and running horribly.  I am guessing they changed the formula but did not say anything.  What can I add to the Matte glaze to raise the melting point of the glaze?  Or at least stop the running?

Glazes are my weak point, whether I have been intimidated by them or just plain too lazy to really understand them.  So any help would be great.

Edited December 13, 2023 by Elmoclayman

[Min]

It's going to be a real crap shoot trying to alter a glaze without the recipe. Which is the glaze that is giving problems, Laguna Turquoise ^10 WC554? Has the amount of reduction you are doing increased?

[Elmoclayman]

Yes!  That's the glaze I'm having trouble with.  Reduction has been the same each firing.  I was hoping there was some material that would increase the melting point.  Like I said, I have no knowledge of how to mix glazes or what materials do what.  I tried adding silica to the glaze because Google told me that it would raise the temp, but I think it did the opposite. ;-)

[Min]

Okay, so in a very simplified nutshell a glaze at any temperature will have 3 main components. Silica (think of this as the bones of a glaze), Alumina (think of this as the flesh of a glaze) and Flux(es) (think of this as the blood of a glaze). All glazes, regardless of cone or firing method, will be comprised of these components. Fluxes are almost always used in combination with other fluxes, ie a glaze will have more than one flux. Silica can be sourced in a glaze as silica and it's also in feldspars etc, a good source of alumina is kaolin or ball clay. 

The higher the cone firing the less fluxes are needed, so by definition there will be room for more alumina and silica in higher firing glazes than lower firing ones. Sounds like your glaze is over-fluxed for the cone you are firing it too.

When you added silica to the WC554 glaze it "did the opposite" I'm guessing it went more gloss? I would hazard a guess this is because you altered the silica:alumina ratio. This can happen when you add silica to a high alumina matte formula. High alumina mattes are glazes where the ratio between the silica and alumina fall within a certain range, adding silica to it will move the glaze into the territory of a semi or gloss. (This works when there are sufficient fluxes to dissolve the extra silica)

What I would suggest doing would be to weigh out a sample amount of dry glaze, say 200 grams, then add both silica plus epk to it. The ratio of the two is important, for every 1 gram of epk you need to add 1.25 grams of silica. Start with adding 10 epk and 12.50 silica to the 200 grams base then mix/sieve and dip a test tile. Repeat adding 10 epk plus 12.50 silica twice more and repeating the test tiles. Fire those and see what happens, in theory it should help stiffen up the glaze and return it to less of a gloss. If it's still too glossy then try with just adding the epk and omit the silica. (or run this test at the same time) Between the 2 tests, one adding both silica plus epk and the other just adding epk I think there's a good chance of success.

If the colour is diluted too much by the silica/epk then address that by adding copper carbonate, have to see how the tests turn out before guesstimating how much of that to add. The purpose of these tests is to "dilute" the fluxes. Like I said this is a crapshoot as we don't have the recipe to work from but I think it's worth trying.

 

Edited December 15, 2023 by Min
clarity

[Elmoclayman]

Thanks so much for the information!  I will try this in the next couple of glaze firings. 

Just thought of another question.  Does glaze "go bad"?  I found some glaze in a tub that says Turquoise on it.  I believe it to be the same Laguna Clay Turquoise.  However, its been in this tub for about 20 years. (yeah, I know we never throw anything out)  Could the glaze 'oxidize" or something like that?  Would it change the way it fires? I haven't tried it, but it does appear to be a different dry color than the new turquoise glaze (dry in powder form).

[Min]

30 minutes ago, Elmoclayman said:

However, its been in this tub for about 20 years.

Wet or dry?

If it's dry then nothing will have changed, if it's wet and a brushing glaze the organics to help with application will probably be shot. Can add some commercial brushing medium or make your own.

Solubles in a dipping glaze could and probably have leached into the water in the bucket if it's still liquid. If this is the case then add a bit of saturated epsom salts solution to re flocculate it. Need to stir it all up and measure the specific gravity, if the glaze seems thin then add a few drops of epsom salts solution to a small amount of the glaze until it's the consistency you want then dip a test tile and go from there.