What Do You Do If Commercial Glaze Runs Excessively?

[azjoe]

I mix most of the glazes I use, but also use a number of commercial cone 5-6 oxidation glazes. Several of the commercial glazes run excessively, others I'd like to melt just a little more. It's not economical to fire the pieces decorated with commercial glazes separately since I never seem to have more than a couple at any one firing. I'm curious how others handle this... do you just not use these glazes, or do you adjust them slightly to raise/lower their melting point slightly? What approach have you found works best?

[bciskepottery]

I make almost all of the glazes I use; the few commercial glazes I use are from Coyote and Opulence. A difficulty with trying to alter a commercial glaze is that you really don't know what is in the glaze and how anything you do to it might affect glaze performance. It could also be an issue of the glaze working fine at ^5 but running at ^6. It could also be a matter of your clay body; some clay bodies have more flux in them and could cause a glaze to melt more than a body with less flux.

 

You could do some test tiles or test cups and glaze them at varying points/thicknesses and try to measure how far the glaze runs and then take that into consideration when using the glaze. You could also try a thinner glaze application on the lower 1/3 of the item and see if that affects the running. But if its a runner, I tend to avoid it -- I have better things to do than scrape kiln shelves. With commercial glazes, its almost always a case of trial and error.

[Matt Katz]

Running glazes are always a result of over-firing.

[azjoe]

About half the glazes I use are products of a major clay/glaze vendor... I purchase them dry in bulk and mix them for dipping according to directions on the vendor's website. They are all spec'd as cone 5 glazes. Some are extremely stable, some sag a little, and several are very runny, no matter what. It's these latter ones I'm asking what people do... other than not use them.

 

Running glazes are always a result of over-firing.

 

Yes, of course they are and I understand the obvious... fire to a lower cone. However, for me that isn't viable since most of my firings are made up of pieces decorated with a variety of glazes that fire as desired at cone 5. Certainly, many glazes need to run a little to properly break and develop their characteristic look... I'm not talking about those... I'm referring to the ones that drip onto the kiln shelf (even when thinly applied) if they're within an inch of the bottom of the pot.

 

My specific question was if/how others may have successfully adjusted the chemistry of a runny commercial glazes to raise its melting point slightly w/o significantly affecting the glaze. I would think adding a little silica would raise the melt point, for example.

[Deb Evans]

FYI, commercial glazes are great when they fire correctly but a pain when they don't. They mix in bulk and sometimes your 25 lbs dry glaze isn't correct....I know of one manufature that never stated they had changed an ingrediant and that the ^6 glaze was now^4 > big difference. Their glaze person wasn't even concerned or interested in retifying the problem.

So, use com glazes untill you can find or create a glaze that will replace it.

[sylvia (UK)]

... or do you adjust them slightly to raise/lower their melting point slightly? What approach have you found works best?

 

 

I was faced with a vast quantity of running glaze when I did a locum at an art centre where the users didn't know they had to stir all the way to the bottom of the bucket. I stirred what was left properly, measured out half a litre of slop, sieved 5g of flint and 5g of china clay into it. (This was a stab at getting somewhere close to 5% of dry ingredients). Was ready to try 20g per litre if necessary but the first stab at it worked a treat.

 

Good luck!

 

Sylvia