Potters & Sculptors - Making Rock from Mud
I bought a 5lb dry mix of electric blue (matte) from glazemixer.com. I loved it but after a few uses I'm having a problem.
When I apply the glaze to a ^05 bisqued piece it cracks and flakes off. If I know I didn't contaminate the glaze as I take a bit out of the main container and then apply onto my bisque.
I've sieved the glaze several more times thinking that would help but it doesn't. Any idea whats going on and how I can correct the problem? Thanks
Thanks for your response George. Since this was my first test of this glaze I had a 5lb dry and mixed the whole batch. I had another potter suggest using CMC gum.
when you say it cracks and flakes off - do you mean before you fire it? or after?
Hi - whoever said Gum - they are correct - see more details about this subject - why re create the wheel....
Now, the question is: What bonds a dry glaze layer to a piece of bisque ware? Well there is no obvious dry adhesion mechanism or boundary chemical reaction that glues glaze particles to the bisque wall. The mechanism of the bond relates to the sticky nature of the wet glaze and the microscopically rough surface of the bisque ware. During hardening the glaze layer loses its wet adhesion and simple mechanical contact is the only microscopic bond. The layer stays on because all the minute surface cracks and pores give it places to hang on to. As you can imagine, this bond is weak at best.
Since all glazes shrink during drying, it is not clear how the weak bond with the bisque is able to withstand the pulling forces associated with the shrinkage. Well some glazes hardly shrink at all because they lack clay content and that is of course why they dust off excessively. However glazes that harden properly during drying always crack, you just do not see the cracks. Micro-cracks must develop to relieve the stress. However when there is too much shrinkage they become visible cracks. With even more stress the glaze cracks to form 'islands' with curled up edges (like a dried up lake bottom). You can see this effect clearly if you watch a slurry of pure kaolin or ball clay dry on a bisque surface.
As you can see, we want a glaze to have enough clay so that it forms a hard dry layer but not so clay that it shrinks excessively and cracks off the bisque. There are a number of strategies you can employ if your glaze is powdering on one extreme or shrinking and cracking off on the other. It follows that a powdering glaze needs either more clay or a finer more plastic clay whereas a glaze that is shrinking and cracking needs less clay or less plastic clay. Typically pottery glazes need a minimum of 20% kaolin (equivalent to 10-15 ball clay or 5% bentonite) to harden adequately.
-If your cracking and shrinking glaze employs a relatively plastic kaolin (like #6 Tile or Sapphire), try switching to a less plastic one like EPK or Pioneer. This will not affect glaze chemistry much. A similar switch of one ball clay for another is not as likely to work since pretty well all common ball clays are very plastic. If your glaze is powdering then switch from the less plastic material to a more plastic one. However I must say that if your glaze has a problem, this one change is not going to solve it, more will be needed.
-Add 3-5% bentonite for powdering glazes, remove it from cracking glazes. Bentonite is super fine and super plastic and therefore dries very hard and shrinks alot. The small amount of bentonite does not affect the glaze chemistry too much. Remember you can't add bentonite to an existing slurry, it agglomerates into balls that even a propeller mixer won't break up; you need to shake it up with the powder in a new batch to separate the particles).
-Add CMC gum to powdering glazes. Like bentonite, it needs to be added during dry mixing. Gum is very sticky and it hardens, using it is a way of 'gluing' a glaze on the ware. Strangely gum also helps suspend, but I have no idea why. Gum burns away so it has no effect on glaze chemistry. One problem: gummed glazes dry slower and drip-drip-drip after glaze dipping pull-out. Experiment with the amount, try 0.5% to start. Do not use gum unless you need it.
see this link for the full story.....
Thanks Patricia. That was an informative article! I will get some CMC gum and test it out. I appreciate the info