Cone 6 Black

[Min]

 

 

Wish I could use them  but the expansion is too high for the pseudo porcelain I'm currently using. Several people in the guild I go to use them on stoneware with success.

Does this mean the same as porcelain shrinks too much for the glaze.

How would one correct this with traditional glazes?

 

 

 

I’m really glad you are interested in this, wish more people were. COE (coefficient of expansion) or CTE (coefficient of thermal expansion) are both terms meaning the same thing.  It is a measurement of how much something expands while being heated and then shrinks while cooling.

If the glaze shrinks more than the clay upon cooling you will get crazing. The crazing can appear shortly after removing pots from the kiln or with delayed crazing it can happen weeks later. Think of a large person putting on a pair of small jeans. The jeans have to stretch to fit, if stretched too much they are going to tear, might not happen as soon as they put them on but at some point the fabric can’t take the stress any more and relieves stress by tearing (crazing).

 

Opposite of the large person small jeans analogy is having your pants fall off (shivering). Dangerous since slivers of glaze will actually pop off the pot, usually on the rims or edges. So in this case a little person is putting on a large pair of jeans, jeans are too big and come off.

Without getting into altering the claybody there are a number of fixes for glaze fit issues. If the crazing is slight (wide spaced crazing) then sometimes the addition of a bit more silica will solve the problem, a line blend is a quick way to do this. Another fix for minor crazing is to fire the pots hotter (if the clay and glaze can take it without blistering or running). The method for more severe crazing (as on low expansion claybodies) is to swap out the high expansion fluxes in the recipe for ones that are lower in expansion. Sodium and potassium are common high expansion fluxes, by replacing part of them with low expansion fluxes such as those containing magnesium or lithium you will lower the COE of the glaze. The addition of some coloring oxides also helps reduce crazing as does Zircopax even though the COE figures will rise.

For shivering it’s the opposite approach, replace low COE fluxes with high COE ones.

 

Clear as mud right? : ) Now it brings up the issue of how do you check for glaze fit…

[yedrow]

Thanks everyone, I think I'm going to give the Roy glaze a try this week (the one from the book). I'll have to look to see where I put the tile from the presumable 'Roy' glaze. If I find it I'll take a pic and post it. It wasn't a cone 6 glaze I'm pretty sure.

Joel.

[Pres]

I had always assumed, wrongly or rightly so, that a metallic looking black was asking for trouble being that it would not be food safe. I kept away from these when I saw them in recipe books and magazines as I wanted my work to be food safe. What about my assumption?

[Guest JBaymore]

I had always assumed, wrongly or rightly so, that a metallic looking black was asking for trouble being that it would not be food safe. I kept away from these when I saw them in recipe books and magazines as I wanted my work to be food safe. What about my assumption?

 

Here's a shocker: At the last "food safe" glaze NCECA talk........ it was specifically stated that all matte glazes are not considered food safe.

 

As to the metallic surface blacks...... I think it is a 'safe assumption' until proven (lab testing) otherwise.

 

(Glass half full.... glass half empty....... is the gun loaded.... and so on)

 

best,

 

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

[Pres]

I had assumed the same of matt glazes. Again wrong or rightly, just that I never believed them to be glassy enough to contain/protect oxides

[oldlady]

ok,  having read all of this long thread and looked at all of the very technical (and scary) formulae (?) I am now more confused than ever.  may I ask a simple question?

 

is Licorice glaze the one that has been dissected here?

 

is there a GLOSSY black glaze for cone 6 that anyone agrees is acceptable for NON FOOD use?

 

thank you.

[bciskepottery]

This started out as a discussion of Ron Roy's Licorice, C6 glossy black; it migrated to a discussion of another black glaze offered by Bob Coyle. 

 

Ron Roy's Licorice was deemed durable and safe in Mastering Cone 6 Glazes; I don't see why you couldn't also use it for non-food uses if you want shiny, glossy black.

[oldlady]

thank you, Bruce.