|Monthly Tech-Tip |
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are issues compared to dipping glaze. You can make your own.
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).
To a potter used to dipping ware and having the glaze dry-to-the-touch in seconds, painting 2-4 layers with a drying period in-between can seem tedious. But in practice it can be practical. The paintability improvement that comes with the gum additions is remarkable, the consistency is not unlike a latex paint. Gums, or mixtures of gums, can both slow down drying and gel the slurry to enable higher water contents.
Organic and inorganic gums are used in ceramics. The former suffer molding issues, so preservatives are needed to counteract this. Brushing media products can also be substituted for part of the water (like propylene glycol, the name sounds bad but its not toxic, or Floetrol, a brand-name product) to make glazes brush better (but they will affect evaporation).
And, you can make your own brushing glazes (see below). Their advantages include:
-A wide range of colors and visual effects.
-Good in classroom settings. Labels are clear.
-Ability to layer.
-Avoidance of the need to mix or maintain larger buckets of dipping glaze.
-The ability to control thickness.
-The avoidance of runny, experimental or miss-mixed test glazes that ruin kiln shelves.
-The ability to glaze unbisqued ware (greenware), re-qlaze or glaze vitrified bisque.
-Food-safe, durable (at least claimed).
-Special effects like metallics, reactives, bright colors, layered variegation.
-Suppliers also love bottled, paint-on glazes. They make more money and inventorying, customer service and labour are reduced.
-The well adhered dense, bubble-free lay-down often fires with fewer defects.
Hobby ceramics brushing glazes were often leaded or high-boron and painted on to unfired green ware. It was practical because almost everyone used the same clay body (50:50 talc:ball clay) and glaze manufacturers knew what to match their products to. Now that paint-on glazes are main-stream at stoneware temperatures, historically-used and commonly-known recipes and effects are being produced by every manufacturer. Commercial glaze lines have brought bright colours at every temperature range, thanks in many cases to inclusion stains. Many lines of glaze are actually just one transparent base with different stains added plus a mixture of gums to harden, suspend and gel the slurry. Some manufacturers even specify the base product and encourage users to make even more variations by adding their own stains.
The disadvantages of brushing glazes include:
-Commercial bottled glazes are much more expensive than mixing your own. A potter stocking up at his supplier can spend thousands of dollars (as opposed to the hundreds he would spend for buying the materials to mix his own).
-Since the body and glaze manufacturers are separate companies, compatibility and fit between the products is often by accident (thus crazing, shivering, going milky are much more common). Typically glaze companies err on the side of a thermal expansion as low as possible, while avoiding crazing this increases the likelihood of shivering and glaze compression failure.
-The recipes are unknown. They claim they are food safe; but very bright colors, high melt fluidity, heavy crystallization; these are all associated with leaching.
-They breed customer ignorance of how glazes work. The recipes and techniques for a wide range of visual effects were once known by potters, and many knew how to tweak them for their circumstances, now this knowledge is being lost.
-They breed an ignorance of how to maintain the specific gravity, viscosity and thixotropy of a slurry.
-They breed a weekend-warrior (as opposed to cottage industry) mentality in ceramics.
-When simple, clean, functional surfaces are needed (transparent or colored) paint-on glazes do not apply nearly as evenly as traditional non-gummed dipping ones. Dipping versions of common bottled glazes are available from manufacturers, but they may be gummed also (which may be because they need to enable painting others over them). These dry slowly and drip, drip, drip more during draining.
-Paint-on is slow. Multiple coats with drying in-between really lengthens glazing time. Doing two thicker coats of higher specific gravity glaze produces much less even coverage (which is really noticeable for bright colors). For the most even coverage possible, four coats of a low-specific-gravity glaze produces the best result.
-Highly gummed glazes do not shed well from wax resist.
-They often do not fire as expected on your clay body (even though the supplier might have glossy and elaborate documentation).
-Stocks of bottles run out more frequently, often at very inconvenient times.
-Old dried-out or water-reduced jars are difficult to restore to working condition and it is almost impossible to sieve them (because of the gum).
There is thus good reason to learn to use both types!
Make Your Own Brushing Glazes
A dipping glaze can be converted into a brushing glaze by employing either a low or high specific gravity approach (the former requiring painting on more coats). Powdered CMC gum cannot be added to an already-prepared slurry, it will not mix in. Simply adding CMC gum solution is not practical either, it needs to be substituted for the same amount or more water (since it may actually thin out the slurry). We have found that 500g of powder, 100g of 7% gum solution (e.g. Laguna) and 250g of water makes one pint with 1.58 specific gravity (which will thicken on standing and need more water late, according to preference.
All of this being said, another philosophy for brushing glazes is to greatly increase water content, driving the specific gravity as low as 1.3. This approach is a favourite among commercial suppliers. Of course, it requires application of more coats. But it also enables painting on each layer more evenly. To do that use a Veegum CER solution, it is a mix of Veegum T and CMC gum, so it slows drying, gels and hardens (that enables the addition of much more water).
The glaze in this jar was 'goop', impossible to paint on because it was too viscous. And it dried way too fast. Laguna mentions adding water so I measured the specific gravity (SG): 1.7. That is super-high, it took a 125cc addition to bring it down to 1.5, but it was still thick, dried even faster and brushing it on evenly was even harder. It was not obvious what to do next. It needed a lot more water (1.3-1.35 SG is normal to support multi-layer application), adding CMC gum and enough water to do that would produce an unusable watery and sticky slurry. Veegum CER to the rescue! It is a 50:50 mix of CMC gum and Veegum T. The former slows drying and hardens, the latter gels. So it can simply be added until the painting properties are right. And, a Veegum CER solution is easier to handle than one of CMC gum. This really worked! The brushing properties are just right and it gels nicely on standing. CER is also good for highly fritted dipping glazes or others lacking in clay content (otherwise CMC might still be better).
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).
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.
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).
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)!
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.
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.
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. However most commercial glazes do have a lower specific gravity (have more water), this does aid further in paintability but requires more coats. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.
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.
These brush-strokes of gummed glaze are painted onto an already-fired glaze (the gum enables adherence and drying without cracking). Notice brush strokes hold their character. The brown glaze on the left 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 flattened and it is running downward. This is not happening because of the lower SG, commercial glazes with an SG down to 1.3 can perform well. How? Because they also have Veegum to thicken them. That reveals thus secret: Gum needs particle surface area to work its magic. We can get that with a bentonite addition (it is fine particled like Veegum). The dried strokes on the right were much better, that glaze adds 2% bentonite. That made all the difference, it painted beautifully.
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.
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 G3890 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 is good. As is obvious, it makes sense to make your own base white. Then 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).
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.
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!
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). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more Veegum). With underglazes it is important to get adequate thickness with one brush-stroke, so a higher 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.
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.
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.
That is less than 50 cents worth of material for an 80% fritted glaze with an expensive stain. Being the independent person you are why not mix that yourself. For the price of this jar you could make 16 times as much.
In ceramics, glazes are developed and mixed as recipes of made-made and natural powdered materials. Many potters mix their own, you can to. There are many advantages.
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.
Base-Coat Dipping Glaze
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.
This term refers to critical thinking ability that potters and technicians can develop to recognize recipes having obvious issues and merit, simply by seeing the materials and percentages.
In ceramics, the edges of overglaze and underglaze color decoration often bleeds into the over or under glaze. How can this be avoided.
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?
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
CMC Gum Calculator - Worksheet
Use this to calculate how much gum and water to use when mixing a new glaze or how to calculate and replace some of the water in an existing glaze slurry with gum solution.