The ceramic hobby casting industry has long used commercially prepared, gummed, paint-on (or brushing) glazes. In recent years the hobby pottery community has followed suit. Even professional potters, who in the past made their own glazes, have embraced the use of pint and gallon bottles of brushing glazes for some or all of their ware. Web sites selling pottery equipment and supplies (and physical stores) often feature glazes very prominently, a reflection of how popular and lucrative these are. In fact glazes are now closer to the centre of the potter's universe than bodies! So clay body manufacturers are adapting their products to work with the glazes (rather than the other way around).
Brushing glazes can go on unevenly for more than one reason
Both of these were glazed by brushing. The inside transparent and white glazes are fairly easy to apply evenly but the bright color on the outside left one certainly is not. The problem is a combination of things. It is difficult to apply it evenly with a brush. It is difficult to get it on thick enough. And this commercial glaze does not contain enough of the purple stain (so I added 6 grams of Mason 6304 Violet stain powder to the 2/3 of a jar I had left, and mixed thoroughly). That, more careful brushing, and an extra layer produced the piece on the right!
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.
I have 161 grams of stain. I need to mix it into how much clear glaze slurry?
Stain powders are expensive. I want to make as much glaze as I can from every gram of this red stain I have at hand. I have weighed a teaspoon of my clear glaze liquid slurry (recipe G2926B). I dried it out under a heat lamp and weighed it again (top left). I have filled those two weights, 8.9 and 4.74, into a spreadsheet I made. It calculates the proportions of water and powder in the glaze slurry. I have filled in "10" for the percent of stain needed. It is telling me I need to mix the stain into 3040 grams of the liquid glaze. That gives me about 5 pints of glorious bright-red dipping glaze. The dipping process enables me to apply it so much more evenly than I can do by paint-on methods (provided that I have the right specific gravity and thixotropy). And, I got this much glaze for about $50 worth of dry materials (vs. $20 for a pint of paint-on glaze).
Common dipping glazes converted to jars of brushing glazes
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity, it works just like the commercial bottled glazes. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.
Stains added to a base glaze can change its melt fluidity. Adjust the base.
At the top is a melt-flow GBMF test ball of a cone 6 satin matte glaze, G2934. Left bottom: 8% 6213 Mason Hemlock green stain added. The color is good but it is not melting as much and the surface is more matte. A solution is to adjust the base: employ a 90:10 or 80:20 matte:glossy blend to give it better fluidity. Right bottom: 8% 6385 Mason Pansy Purple stain added. The percentage of stain appears to be a little low and its surface is a little too matte. Again, blend a some glossy clear in the the matte base to shine it up a little.
Dip-glazing vs. brush-on glazing: Which gives the more even surface?
This is a clear glaze (G2931K) with 10% purple stain (Mason 6385). The mugs are cone 03 porcelain (Zero3). The mug on the left was dipped (at the bisque stage) into a slurry of the glaze (having an appropriate specific gravity and thixotropy). The glaze dried in seconds. The one on the right was painted on (two layers). Like any paint-on glaze, it contains 1% CMC Gum. Each layer required several minutes of application time and fifteen minutes of drying time.
Commercial glazes may not work on your clay body
Left: Plainsman M390. Right M370 porcelain. The bottom two samples are a popular ultra clear commercial bottled glaze that costs about $13/pint. On the porcelain, it is crazing. On the red clay it is saturating with micro-bubbles and going totally cloudy and even a satin surface (it should be like the transparent above it). It is likely very high in boron and melting too early. Whose fault is this? No ones. This glaze is simply not compatible with these two bodies.
Brush-on commercial pottery glazes are perfect? Not quite!
Paint-on glazes are great sometimes. But they are even greater if you know the recipe, then you can make more and make a dipping version for all the times when that is the better way to apply. Why is that better? Because you have a huge advantage over a glaze manufacturer: You already have clear glossy and matte base recipes that fit and work on your clay body. You can add the stains and opacifiers to these (with 1% gum to make them paintable) and make your own jars. Don't have base recipes??? Let's get started developing them with an account at insight-live.com (and the know-how you will find there)!
My clear glaze outside. Commercial white inside. But a big problem!
I know my outside glaze recipe fits this terra cotta. It does not shiver on sudden heating or craze on sudden cooling. And I have a gallon so I can dip-glaze the outside and it dries perfectly in seconds. But that inside glaze? It is under too much compression, so much so that it is literally forcing the piece apart (that crack exploded onto the scene with a loud ping a day after firing). But I do not know the recipe. And I had to paint it on in three coats. The painting was difficult and it took ten minutes to dry each coat. A better way to do a cover glaze would have been to simply add 10-15% Zircon to my clear recipe (I can even adjust if the added zircon lowers its expansion too much). To apply that would have been a simple pour-in and pour-out. Or I could make my own pint-jar of brush-on by using a mix of gum solution and water (instead of pure water).
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.
Specific gravities on three commercial glazes might surprise you
The freshly opened transparent low fire glaze on the left has a specific gravity of only 1.34 (that is a high water content). Yet it is viscous because they add alot of gum. It needs three coats to go on thick enough and takes quite a bit of time to dry each one. When dipping, a very thick layer dries very slow and thin. That being said, transparent glazes do need to be applied thinner, AMACO is likely trying to assure that by producing at this low specific gravity. The center Potter's Choice glaze, made by the same company, is 1.52 (that is a much better deal). And it goes on nice and thick. The Celadon glaze on the right is lower, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they just adapt the percentage of gum to impart the viscosity they want. While it is sensible to use commercial special-effect paint-on glazes, clear cover glazes are best mixed yourself and applied by dipping or pouring.
Using commercial glazes? You still need to know about specific gravity.
The glaze in this jar was 'goop', impossible to paint on. I did not know whether I needed to add water or try to deflocculate it (although the former is more likely and in keeping with what Laguna says on its website). I measured the specific gravity, it was 1.7, so clearly it needed water. It took 125cc to bring the specific gravity down to 1.5. However, it was still thick and dried immediately after painting on, clearly it does not contain enough gum for brushing. The next time I will add a mix of 50:50 gum solution and water for better paintability. The bright side: I got considerably more than a pint after adding the water, a big difference from some other commercial glazes which are mostly water.
Gum does not work in a glaze if an important ingredient is missing
These brush-strokes of gummed glaze are painted onto an already-fired glaze. Gummed glazes can do this, they will adhere and dry without cracking. And dry hard and resist washing off. Brush strokes hold their character. The brown glaze has 1.6 specific gravity (SG) and about 1.5% CMC gum. The white one has the same gum content but an SG of 1.5. It's brush stroke has flowed flat and it is running downward. Is it because of the lower SG? No. Commercial glazes with an SG down to 1.3 perform well also. The secret: Gum needs particle surface area to work its magic. We can get that with a bentonite addition. The dried strokes on the right were much better, that glaze adds 2% bentonite (and we raised the SG to 1.6). That made all the difference, it painted beautifully.
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.
Liquify of a pint of brushing glaze. It's easy!
I counter-balanced the measuring cup and weighed out 250g of water. Then I added 100g of Laguna gum solution and stirred it. I put that into the blender and added 500g of powdered glaze (you can use any glaze recipe). I started the blender on slow then increased it one-at-a-time to full speed. After less than a minute (and a little work with a spatula) it was creamy smooth. It painted evenly on the tile just like a commercial bottled glaze, drying slowly. This produces a specific gravity of 1.58 (which is pretty high) so I can add water and thin it with no issues.
The specific gravities on two commercial underglazes might shock you
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats, and more time to dry and apply them). Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (but thinning down as it dries). With underglazes it is important to get adequate thickness with one brush-stroke, so a high specific gravity is important. This may be reason enough to consider making your own (by adding stain powders to a base). Actually, this technique of adding-stains-to-a-base-transparent is even more practical for making your own glazes, it just takes the right amount of gum to make them paint well.
Black and white cone 6 brushing glazes were easy to make
We started by adding 500 grams of the G2926B Whiteware base clear to 250g of water and 100g of Laguna gum solution. It was possible to stir all the powder in without a mixer. There were a few lumps left but they broke down overnight (yielding about 550 ml at 1.58 specific gravity). For the black we added 30g more of Mason 6666 stain (6%) and for white 50g of zircopax (10%). This increased the specific gravity to 1.63, higher than pretty well any commercial brushing glaze (if needed, there is plenty of room to add water to thin it for better application properties). The black recipe costs about 1.37 cents/ml for us to make (compared to Amaco C-1 Obsidian @ 3.92 cents/ml to buy). But the situation gets even better: If we were to add enough water to bring the specific gravity down to the 1.4 of C-1 there would be far more than 550ml.
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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