|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 also issues compared to dipping glazes. You can also make your own.
Key phrases linking here: brushing glazes, brushing glaze, high sg, low sg - Learn more
The ceramic hobby casting industry has long used commercially prepared, gummed, paint-on (or brushing) glazes and underglazes. 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. Websites selling pottery equipment and supplies (and physical stores) often feature glazes very prominently, a reflection of how popular and lucrative these are. In fact, prepared glazes are now closer to the centre of the potter's universe than clay 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 latex paint. Gums, or mixtures of gums, can both slow down drying and gel the slurry to enable higher water contents and better hold-in-place.
We talk about brushing glazes in terms of 1 pint or 500ml jars. Filling a jar with a high specific gravity, or high SG, 1.6 slurry could be 500:350g of powder:water. A low SG 1.3 version could be the opposite, 500:350g. So first you must decide which to target. The high SG approach is better for slurries having very high percentages of plastic clay (e.g. underglazes, slips) they need to go on thicker in one stroke (especially on leather hard clay). Brushing glazes for bisque actually work well with much lower SG’s (made possible by a Veegum addition), these require more coats but give better control of total thickness and hang on better on vertical surfaces.
Organic and inorganic gums are used in ceramics. The former suffer bacterial growth issues, so antimicrobials are needed to counteract this (bacteria actually degrades the performance of the gum). Brushing media products can also be substituted for part of the water (like propylene glycol, the name sounds bad but it's not toxic, or Floetrol, a brand-name product) to make glazes brush better. Actually, you can make your own brushing medium that can be added to any dipping glaze by mixing Veegum CER and water. Beware of any product that bills itself as a additive that can make any glaze brushable because three properties are needed: Slower drying, hardening on drying and gelling to prevent settling and improve brushing behavior. The level of each depends on the percentage and nature of clay in the recipe.
The advantages of brushing glazes 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.
-Greater 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 began as paint-ons to unfired greenware. It was practical because almost everyone used the same clay body (50:50 talc:ball clay) so glaze manufacturers knew what to match their products to. Now that paint-on glazes are mainstream at stoneware temperatures, commonly-known recipes and effects are being produced by every manufacturer. Commercial glaze lines also feature bright colours at every temperature range, thanks in many cases to inclusion stains. Many glaze lines are actually just one transparent base with different stains added plus the mixture of gums to harden, suspend and gel the slurry. And a biocide to prevent bacteria from attacking the gums. Some manufacturers even specify the base product and encourage users to make even more variations by adding their own stains or mixing the colors.
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 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 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 (to function as a base coat). 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 produce the best result.
-Highly gummed glazes do not shed well from wax resist.
-They often do not fire as expected on a 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
Any dipping glaze can be converted into a brushing glaze. But it is not a matter of adding powdered gums or bentonite to an already-prepared slurry, they will not mix in (unless you are blender mixing). True, to make a high specific gravity slurry it can be effective to simply add CMC gum solution to an existing dipping glaze. However, the best approach is to clearly consider the amount of each kind of gum, bentonite and water for each recipe and mix-from-scratch each one.
Second, decide whether to add bentonite, adequate particle surface area is needed on which the gums can act. If a glaze already has plenty of clay, especially ball clay, then bentonite is not needed (note that Gerstley Borate is also a plastic clay). But if it just has 20% or less kaolin then bentonite will be needed (e.g. 2%). At the other extreme, underglazes will certainly need bentonite if the recipe does not have a very high clay content (e.g. more than 60%). Bentonite is inexpensive and may minimize the amount of Veegum needed later.
Third, CMC gum will always be needed to slow drying and harden and adhere it. About 1.5% of the total powder amount is a good starting point. In the past, we have added it via 7% gum solution but blender mixing of the powder has proven to be both much easier and just as effective. Something important to consider is microbial attack on the gum over time, this becomes evident if the glaze goes thinner and does not cover or paint on as well.
Fourth, glaze slurries, which will almost always be low SG must be gelled to prevent settling and keep them from running after application. After blender mixing and test brushing it will be clearly evident if VeeGum will be needed. The powder can be slowly added while the mixer is on high speed until the degree of gel needed is reached, the amount should not be over about 1.5% of the total powder weight. As a guide we use 5g each of CMC and VeeGum in 340g powder and 450g of water.
Veegum CER, a 50:50 mix of Veegum T and CMC gum, is the way to do this, use about 3%. This addition slows the drying, gels the slurry and hardens the layer. In the past, we have recommended using a gum solution but now find that a home or industrial blender is far better (just mix the gum powders with the rest of the glaze powders and add that to the needed amount of water and process batches of that in the blender to gel it). With this approach, no sieving is needed.
For brushing engobes a CMC-only approach is likely better (since they have high clay content and thus suspend and gel naturally). That being said, if the engobe contains a granular (e.g. manganese) then extra gelling is needed and some Veegum, perhaps half of the above mentioned, would be good.
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 for multi-layer application of low SG glazes), 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. The brushing properties are just right, it gels nicely on standing and stays in place on verticals. CER is also good for highly fritted dipping glazes or others lacking in clay content (otherwise CMC might still be better).
To make a low SG version of G2934BL I have already weighed out a 340g batch (it contains 5g each of Veegum and CMC gum to gel the slurry and slow the drying). I use 440g of water initially (adjusting that according to experience in brushing behaviour). After shake-mixing all the powder in the plastic bag I pour it into the water on low speed and finish with 20 seconds on high speed. This produces a low specific gravity brushing glaze, it just fills this 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel and the CMC for slower or faster drying. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Since each glaze recipe responds differently and changes differently over time, good notes are essential. We are working on dozens of these at any given time, each is code-numbered in our group account at Insight-live.com. This is so worthwhile doing that I make quality custom labels for each jar!
To make high SG brushing versions of these glazes I started by blender mixing 500 grams of the G2926B Whiteware base clear to 250g of water and 100g of Laguna gum solution. That yielded about 550 ml, the resulting 1.58 specific gravity makes each layer go on quite thick. 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, much higher than any commercial brushing glaze. 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). The price advantage would be much better if we were to add enough water and Veegum gelling agent to bring the specific gravity down to the 1.4 value of C-1.
This jar of glaze will do seven of these mugs! Four coats are required because it is watery. What about the time? To glaze a thin-walled piece like this could take an entire morning of applying coats and waiting for them to dry, more time than actually making the piece itself. There are two obvious choices for a more economical and faster method: Make your own high SG brushing glaze and do it in two or three coats, heating the piece to about 200F between each. Or heat the piece once to 250F and quickly immerse it in a dipping glaze and be done in 10 seconds! Our G3879 recipe is a good starting point for both options.
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats gave very thick coverage). Recipes are GA6-C Rutile Blue on the outside of the left mug, GA6-F Alberta Slip Oatmeal on the outside of the center mug and GA6-F Oatmeal over G2926B black on the outside of the right mug). These are examples of high specific gravity brushing glazes. One-pint jars are made using 500g of glaze powder, 280g of water and 75g of Laguna CMC gum solution (equivalent to 1%). Because no Veegum is being used this blender mixes to a slurry of super high 1.6 specific gravity (SG). Commercial glazes have a much lower specific gravity (thus much more water), giving better paintability and gelling but requiring more coats. Still, this approach is good for Alberta Slip because it is highly plastic and comprises the bulk of the recipe. The gum removes the need to roast 50% of it and the plasticity of the Alberta Slip helps suspend the slurry.
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 underglaze decoration, it is important to get adequate thickness with one brush stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow down its drying and harden it well at the dried state).
The freshly opened transparent low fire glaze on the left has a specific gravity of only 1.34 (thus has a high water content). Yet it is viscous and holds in place 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. For low SG glazes like this dipping is pretty well impossible. That being said, highly fritted transparent glazes do need to be applied thinner and they actually do not work well at higher specific gravities, AMACO is right to produce it at this low specific gravity. The center Potter's Choice glaze, made by the same company, is 1.52. And thus goes on nice and thick with each coat (and is thus a better deal). That glaze likely contains lots of clay so little or no gelling agent (e.g. Veegum) is needed. The Celadon glaze on the right is in between, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they adapt the percentage of gum (e.g. Veegum and CMC gum) to the nature of the powdered materials in the recipe to impart the needed rheology and brushing characteristics.
Only a part of that 21 grams is actual stain powder, it is mixed into a medium. A 16oz jar of another color yielded 190g, a slightly better percentage of powder-to-water. The high water content is actually a benefit, they condition it with a clay that causes it to gel - these two factors produce better painting properties. In one scenario we calculated a 1500% saving in making your own vs. buying these jars, that is a more feasible idea than you might think.
These were fired at cone 04 using Spectrum glazes. Both were glazed using the same brushing technique. The one on the left has 4 coats, on the right three coats. The high spots on the throwing rings and the edges of contours are more visible when the glaze is thinner.
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!
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 brush-strokes of gummed glaze are painted onto an already-fired glaze (the gum enables adherence and drying without cracking). Notice the 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. Its brush stroke has flattened and it is running downward. Is this because of the lower SG? No - commercial glazes with an SG down to 1.3 still hold in place well. How? Because they also have Veegum to thicken and gel them. That reveals a secret: Gum needs particle surface area to work its magic. A bentonite addition supplies that. The dried strokes on the right demonstrate that - 2% bentonite has been added to that one.
Left two: Plainsman M390 stoneware. Lower right: M370 porcelain. The bottom two samples are a popular cone 6 ultra clear commercial bottled glaze that costs about $20/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 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. The one on the upper left has almost no bubbles and no crazing. It is the GA6-B recipe and is well documented and easy to adjust.
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 gums 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 gum (1% CMC). Each layer required several minutes of application time and fifteen minutes of drying time. Yet it is still not evenly applied.
This wheel-thrown bowl is large, heavy and thick walled. There are pouring techniques to apply different colored dipping glazes to the inside and out but they are cumbersome and slow. It turns out that the hobbyist way of glazing is actually better - by brushing. I make these 500ml jars of brushing glaze and tune the percentage of gums to get the painting consistency and drying time best for me. Applying multiple coats by brush took only a few minutes and no wax resist or any other tools were needed. And it was easy to meet the two glazes in a straight line at the rim using the brush. The inside glaze is G2936B and outside is G2926B.
Learn to mix any of your glazes for these three application methods
Control gel by adding Veegum as the glaze is mixing
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 ceramic industry glazes are often sprayed, especially in sanitary ware. The technique is important.
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?
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.
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.
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.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work or bottled expensive glazes. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and drying properties. Even some chemistry.
CMC Gum Calculator - Worksheet
CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness.
|By Tony Hansen|
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