These are ceramic glazes intended for dipping but which contain a gum to enable them to adhere to the body better and tolerate over-layers without danger of flaking or cracking.
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).
Some glaze recipes contain materials that make them hostile to multilayering. Materials that cause bubbling, for example magnesium carbonate or spodumene could be problematic. Materials that are partially soluble can make a slurry gel over time requiring the addition of water, this causes more shrinkage and cracking. High percentages of clays cause excessive shrinkage also. Glazes have too little clay and thus dry powdery are also troublesome.
In recent years potters have become increasingly interested in layering glazes for visual affects. The commercial bottled products they use contain gum, so they can layer over each other without danger of flaking or cracking off. So obviously the base coat over which these are applied needs to be able to stay adhered well to survive being rewetted with each new overglaze. Turning a regular dipping glaze into one intended for over-laying will mean the some sacrifice of fast-drying and even coverage possible (depending on the amount of binder you add).
This bowl was dipped in a non-gummed clear dipping glaze. Such glazes are optimized for fast drying and even coverage. However their bond with the bisque is fragile. The blue over-glaze was applied thickly on the rim (so it would run downward during firing). But during drying, it shrunk and pulled the base coat away at the rim (likely forming many tiny cracks at the interface between the clear and the bisque. That initiated the cascade of crawling. When gummed dipping glazes are going to be painted over, a base-coat dipping glaze should be used. What is that? It is simply a regular fast-dry dipping glaze with some CMC gum added (perhaps half the amount as what would be used for painting). There is a cost to this: Longer drying times after dipping and less even coverage. And gum destroys the ability to gel the glaze and make the slurry thixotropic.
The mug on the left has three coats of Spectrum Majolica base, painted on by brush. 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 coverage are good. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (e.g. 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).
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.
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.
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 a cover or a liner glaze 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.
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.
In traditional ceramics and pottery dipping glazes can be of two main types: For single layer and for application of other layers overtop. Understanding the difference is important.
In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats?
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. But there are also issues compared to making your own.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.