Post 153 Glaze Crawling

I'm going to share a learning with you about glazing; one I've not experienced before and hope not to again.

My usual practice when I'm glazing pots is not to do a firing with all the one glaze on many pots. That means I mix up a number of separate batches for the different pots. With a precision scale this is possible. Because I paint the glaze on the pots I need a repeatable way to know I've got enough on. The system I've worked out in true engineering fashion is to estimate the surface area of the pot and then apply the equivalent of about 400 to 500grams of dry glaze ingredients per square meter of surface. That usually takes four or five coats of glaze.

When you get familiar with the characteristics of a particular glaze formulation this all works its way out into a repeatably successful model.

With a request for a big oval pot in a bright glossy blue glaze well of course the answer is 'sure no problem'. I had trialled a glaze a long time ago and was impressed with the colour on the test tile; just what was needed.

Because it was going to be a glossy pot I wanted to be very sure that the glaze was applied evenly and thickly enough to be certain of a uniform surface. So I mixed up a little extra and applied a little extra, as it turns out just a little extra too much. I'd never really experimented to understand how much was too much and do know that a big pot is the wrong place to run such an experiment.

This is what the pot looks like from the good side.

The closeup shows the surface better.

And then you get to the back side. It is amazing that the surface tension of the molten glaze overcomes the adhesion forces and just pulls the glaze along the surface of the pot, in some places defying gravity. Here we have classic glaze crawling, so aptly named. It looks like it has exploded off the surface but the shelf was entirely clean.

The moral of the story, well two actually; the first is that there are always surprises just around the corner to trip you up along the path of learning and more specifically when you have worked out how much glaze to apply don't mess with a successful formula and put too much on.
Now please excuse me, because having defied the kiln gods, I have to go and make another big oval pot, that needs to be glazed in a nice bright shiny blue glaze!

If you are interested in the formula for this glaze it is as follows:

Custer Potash Feldspar	40.0
Silica	23.4
Ball clay	9.5
Talc	6.7
Bentonite	2.2
Gerstley Borate	18.2
TOTAL	100.0
Cobalt carbonite	2.2
Zinc oxide	8.6
Strontium Carb	5.4

Next time I use this one I'm inclined to cut the silicon to say 18 and the zinc to 5 or 6. This will still deliver a good glossy glaze, but the original formula will work just fine - just not too thickly.