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.
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.
CMC Gum is magic for multi-layering, even for raw Alberta Slip
The glaze on the left is 85% of a calcine:raw Alberta Slip mix (40:60). It was on too thick so it cracked on drying (even if not too thick, if others are layered over everything will flake off). The solution? The centre piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. How is this possible? 1% added CMC Gum (via a gum solution)! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied. No cracks. CMC is the secret of dipping-glazes for multi-layering. The down side: More patience during dipping, they drip a lot and take much longer to dry.
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.
How to convert a dipping glaze to a brushing glaze
I have a jar of testing clear glaze that I mixed myself (10% yellow stain and 2% zircopax added to cone 03 G2931K clear). Commercial glaze producers make their lines of glazes like this. The cost of the dry materials: About $6. How can I make it paintable? I made a spreadsheet where I can specify the weight of the plastic jar, the percentage of CMC gum powder needed and the concentration of the gum solution. I just need to weigh the jar of glaze (without lid), weigh a teaspoon of the liquid glaze (lower left), dry it (upper right) and weigh the dry (lower right). After filling in these numbers the sheet tells me what weight to evaporate the jar to and how much gum solution to mix in. It paints on just like a commercial glaze. But don't do this. I made another spreadsheet online (link below) based on starting from dry ingredients, adding the correct amount of water and gum solution. Of course, you need a good mixer to do this.
Better to mix your own cover glazes for production?
Yes. In this case the entire outside and inside of the mug need an evenly applied coat of glaze. In production, it would not make sense to attempt this by painting. For these reasons: Cost, quality, convenience. The right pail has 2 gallons of G2934Y base with 10% Cerdec yellow stain: $135. Cost of jars with the same amount: Almost $300! And you have to paint them on in three coats with drying in between. The one in the pail is a true dipping glaze (unlike dipping glazes sold by glaze manufacturers that dry slowly and drip-drip-drip just like brushing ones). This one dries immediately after dipping in a perfectly even layer (if mixed according to our instructions). And a bonus: This pail can be converted to brushing or base-layering versions using CMC gum.
Can you brush a dipping glaze on to leather hard ware? No.
The body needs to shrink as it dries. Typical glazes have low clay content and shrink very little. So as the body shrinks underneath the glaze just flakes off. Brushing glazes contain significant amounts of gum, that gum bonds them securely to bisque ware, but not to unfired ware. As you can see here. the glaze bond with the body could not withstand the differential during drying.
Mason stains in G2934 matte base glaze
These are Mason stains added to the cone 6 G2934 silky MgO matte liner base glaze (with tin, zircopax and various stains added). The brightest colors (6600, 6350, 6300, 6021, 6404) were tested overnight in lemon juice without visible changes. It is known that MgO mattes are less prone to acid attack that CaO mattes. Caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.
Mason stains in a cone 6 clear base
These are Mason stains added to cone 6 G2916F clear liner base glaze. Notice that all of these stains develop the correct colors with this base (except for manganese alumina pink 6020). However caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.
A matte and a glossy liner glaze
Left: Ravenscrag G2928C matte on inside of mug. Right: A clear glossy. The matte needs to be soaked in the kiln long enough to make sure it develops a functional surface, especially on the bottom. Mattes are not always the best choice for food surfaces, but you can do it if you blend in enough glossy glaze to make it smooth enough not to cutlery mark.
Commercial glazes on decorative surfaces, your own on food surfaces
These cone 6 porcelain mugs are hybrid. Three coats of a commercial glaze painted on outside (Amaco PC-30) and my own liner glaze poured in and out on the inside (G2926B). When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company), without crazing or shivering, it is purely an accident! So use them on the outside. But for inside food surfaces make or mix your own. When you know the recipe you can tune the thermal expansion. And the degree of melt. And the application properties. And you can use quality materials to source a balanced chemistry. The place to start understanding your glazes, organize testing and development and document everything is an account at Insight-live.com.
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.
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