Potters & Sculptors - Making Rock from Mud
Have you tried glazing the pots at a little past leather hard? Might help with the crawling.
Another thing you could do is calcine a good % of the clay content in your glaze. Maybe start with 10% of it and go from there. This would remove a lot of the crawling effect and allow you to get it on thicker as well if needed.
Alumina isn't the same chemistry as clay / calcined clay. We buy Imerys calcined kaolin in 50 pound bags sold under their Glomax brand name.
The only trade-offs are:
1.) You use 12.5% less calcined kaolin than kaolin because the LOI (loss on ignition) has already occured;
2.) Calcined kaolin is physically like alumina or frit in that it has no ability to suspend a glaze, so you still want some clay - 10% or so.
3.) Some raw materials for glazes like Pioneer Talc are sold with natural carbon or other inclusions. Calcining-off these impurities can result in particles whose smaller weight and size become an inhalation risk which didn't exist in the material as shipped.
There are alternate solutions to glaze or underglaze which doesn't adhere well to clay:
A.) reformulate the glaze to add low temperature "melters" such as a frit or borate which melts at a much lower temperature which acts as a glue in the early firing stages. This is the reason I added Gerstley Borate to our studio made underglaze recipes.
Too much though can make the glaze molten prior to the clay body off-gassing leading to foaming and pinholes if the glaze doesn't have a long enough hold at the peak temperature to lay down;
This is why it's a good idea to fire test tiles of a new Cone 6 glaze in a bisque, in order to see to what degree the glaze is melted by that temperature. We've found some "Cone 6 glazes" are not only mature at Cone 04, but actually preferable in color and other characteristics, so we relabel them as a low-fire glaze. Powderbox Pink, a Boron Tin Pink is an example of this. At Cone 04 it is melted and quite pink. By Cone 6 enough of the tin is gone that the glaze is far more pale.
B.) reformulate the glaze so the dry glaze is more porous, easily allowing off-gassing from the clay through the layer of dry glaze;
C.) Add an organic gum or other binder to the glaze to keep the glaze layer intact prior to firing. This burns off around 250 C, well below the point at which the unbisqued clay body or bisque would begin to off-gass;
D.) make sure your glaze is not crawling due to excess magnesium, sometimes from Epsom salt usage, or other problems like dust or oil on the bisque or greenware.
E.) another problem is associated with "buckets of glaze" which change chemistry over time when people don't mix the glaze prior to use. The result is they initially use up the less dense elements of the glaze, later on using a glaze which contains only the denser ingredients. Neither partial fraction of the glaze turns out as intended.
Tom Humphries said:
Thanks Joseph, I did try glazing one at past leather hard and had similar results. I am aiming to get a glaze working for bone dry pots, I think I'd prefer glazing everything together at that stage.
I've had a read about calcining clay, maybe I will try that. It seems like more work than adding a little calcined alumina to replace a clay reduction but maybe the results will be better.
Tom - If the glaze has fine clay, especially bentonite / bentone, it will dry to a hard non-porous layer which off-gassing from greenware has to breach in some way. Either through an unglazed area, or slowly, or quickly by blowing-off the dried glaze in areas.
I know it's an odd thing to do, but if you really want to see what a glaze on greenware combination has done by 550 C or 620 C, simply fire to that temperature. The next day you can handle the test pieces to really see what has happened as they go through the kaolin to meta-kaolin conversion.
Peepholes are good for something, but I'm working with warm glass currently. Even when opening the kiln lid fully at 625 C to see how soft the glass has become, which would be unthinkable for ceramic, there's a limit to how much I can see clearly as the heat distorts what things look like - and it's not the same as touching something.
I've always done a "screen capture" of the screen with Insight-Live, cropped and sized the image in Photoshop then uploaded the resulting JPG but I never knew why. Now I know the much simpler "copy and paste of XML" wouldn't have worked well.
But my answer would be do a test firing to the 620 C or so where the primary off-gassing has completed. It's tough for me to judge how impermeable a dry glaze will be by looking at a recipe.
Tom Humphries said:
Copying pasting XML recipe from insight to here doesn't seem too work