Base-Coat Dipping Glazes
In ceramics and pottery dipping glazes can be of two main types: For single layer or as a base for the application of other layers overtop. We call the latter "base coat" dipping glazes. When other layers of glaze are to be dipped or brushed over a dipping glaze (and no firings are done between layers), then that dipping glaze needs to contain a binder (usually gum) to fix it well to the body and harden it on drying. Without the binder, successive layers pull the first away from the body. Binders are much more effective when the glaze has plenty of super-fine particles for them to bind with (thus bentonite additions are common in gummed dipping glazes). These glazes dry slower, go on thinner and drip a lot during draining (compared to traditional single layer dipping glazes).
Commercial brushing glaze on a non-gummed dipping glaze: Crawling
Non-gummed dipping glazes go on evenly and dry quickly on bisque ware (if properly gelled). But they only work well as a single layer. If you try to paint commercial gummed brushing glazes over them the latter will compromise their bond with the body, cracks will develop during drying and bare patches like this will result during firing. For multi-layering the base dipping glaze must be gummed (e.g. 1% CMC gum). It will go on thinner, drip longer and dry much slower, but that is the price to pay if you want to layer over it.
Six layers, 85% Alberta Slip in the glaze, yet no cracking. How?
Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. Yet this one is 85% clay, it shrinks so much that it would be like a "dried up lake bed" on the first layer. How was it possible to dip it on in six layers here? 1% CMC gum (via a gum solution). This is incredible! Typical dipping glazes contain only about 20% clay (plus things like feldspar, frit, dolomite, calcium carbonate and silica) and they dry within seconds and work well for single-layering. But for multi-layering these it is also vital that gum be added. There is a down side: A drying period is needed between each layer, the length depends on the porosity and wall thickness of the ware. One more thing: VeeGum is not a substitute for CMC, it is a highly plastic clay that will make flaking even worse.
Think the idea of mixing your own glazes is dead? Nope!
These are two pallets (of three) that went on a semi-trailer load to a Plainsman Clays store in Edmonton this week. They are packed with hundreds of bags of powders used to mix glazes. More and more orders for raw ceramic materials are coming in all the time. Maybe you are using lots of bottled glazes but for your cover and liner glazes it is better to mix your own. And cheaper! And there are lots of recipes and premixed powders here to do it. One of the big advantages is that when you dip ware into a properly mixed slurry it goes on perfectly even, does not run and dries on the bisque in seconds. No bottled glaze can do that.
Powdery glaze needed gum solution. How?
I weighed the bucket of glaze and then put about a third of it on a clean plaster surface to dewater. Then I put that back in the bucket and weighed it again. It was 400 grams lighter so I added 400 grams of Laguna Gum Solution. That changed it to a drip-drip-drip dipping glaze but it now dries much harder and I can do thick brushwork on it without worrying about it lifting off.
For even coverage white majolica glazes must be applied by dipping
The mug on the left has three coats of Spectrum majolica base, painted on by brush from a pint jar. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness is good and I got very even coverage. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (I use the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine tune the fit with the body (a big issue at low fire).
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