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Brushing Glaze

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

Details

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 an 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.

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.

Related Information

You may know Veegum T but do you know VeeGum CER?

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Attempts to use a glaze that is too thick

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).

Here is my setup to make brushing glazes and underglazes by-the-jar

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Glaze mixing setup

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!

Two methods to make your own low SG brushing glaze.

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500ml jar of brushing glaze

Dipping glaze applies and dries in seconds. Brushing glazes dry slowly and dry hard. Brushing glazes simply have gum in the recipe, dipping glazes typically do not. This is the Alberta Slip cone 6 base, made jet black with 4% black ceramic stain (our code G3914A). We normally mix this as a dipping glaze but I have made a 500ml jar of low SG brushing version using both of the following methods. These methods will work for almost any glaze recipe (for those having exceptionally high clay content less Veegum is needed). 1) Shake together, in a plastic bag, 340g of mixed glaze powder with 5g of Veegum and 5g of CMC gum. Add that to 440g of water in a kitchen blender and mix on high speed until it gels (the gums resist mixing so the highest speed for at least 30 seconds is needed to prevent lumps). 2) Take 680g of dipping glaze (assuming it is about 50:50 water:powder you get 340g of dry), put that in the blender with another 80g of water and proceed as in method 1. Less total water is being used because the dipping glaze might not be exactly 50:50 water:powder. During mixing, if it gels too much add the extra 20g of water.

Black and white cone 6 brushing glazes were easy to make

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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.

The glaze cost on this mug is three times the cost of the clay!

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Transparent glazed mug and jar

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. There are two obvious choices for a more economical and faster method: Make your own high SG brushing glaze and do it in two 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 or G1916Q recipe families are a good starting point for both options. Another point about time cost: We can weigh out and mix a jar of brushing glaze of either in ten minutes. Or weigh out a whole pail of a dipping version in 15 minutes.

Common dipping glazes converted to jars of high SG brushing

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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.

Specific gravities on three commercial glazes might surprise you

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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 a lot of gum. It needs three coats to go on thick enough and takes quite a bit of time to dry each one. Highly fritted transparent glazes need to be applied thin enough to be clear, thick enough to be glassy smooth but not too thick (to avoid clouding). Amaco likely targets the low specific gravity to enable good control of thickness. The center Potter's Choice glaze, is 1.52. And thus goes on nice and thick with each coat. 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 impart the needed rheology and brushing characteristics.

Very low specific gravities on two commercial underglazes

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Commercial underglazes with low specific gravities

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 gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with eachbrush 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).

A 2oz jar of underglaze dries down to 21g of powder. That is bad and good!

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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 be thixotropic (gels). In one scenario we calculated a 1500% saving in making your own underglaze vs. buying these jars, that is a more feasible idea than you might think. With out setup we can weigh and make a 1 pint jar of brushing glaze/underglaze in ten minutes.

Dip-glazing vs. brush-on glazing: Which gives the more even surface?

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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.

Brush-on commercial pottery glazes are perfect? Not quite!

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Brushing glazes are great sometimes. But they would be even greater if the recipe was available. Then it would be possible to make it if they decide to discontinue the product. Or if your retailer does not have it. Or to make a dipping glaze version for all the times when that is the better way to apply. The glaze manufacturer did not consider glaze fit with your clay body, if they work well together it is by accident. But if you have transparent and matte base recipes that that work on your clay body then adding stains, variegators and opacifiers is easy. And making a brushing glaze version of any of them. 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!

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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).

Commercial glazes may or may not work on your clay body

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Crazing, bubbling pottery glaze

Left two: Plainsman M390 stoneware. Lower right: M370 porcelain. The bottom two samples are a popular cone 6 ultra clear commercial bottled glaze. On the porcelain, it is crazing. On the red clay it is saturating with micro-bubbles and going totally cloudy, with so many ripples in the surface the texture has become satin. 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. It is well-documented and easy to adjust.

Gum does not work in a glaze if an important ingredient is missing

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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.

Commercial brushing glaze on a non-gummed dipping glaze: Crawling

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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.

Brushing glazes can go on very unevenly, adding stain can help

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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!

4 coats vs 3 coats of red brushing glaze

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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.

Glazing large bowls inside-and-out using brushing glaze

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Large bowl glazed by brushing

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.

Inbound Photo Links


Three containers of glaze slurry
Learn to mix any of your glazes for these three application methods

Mixing a brushing glaze
Control gel using Veegum, brushing properties with CMC gum

Ravenscrag Floating blue vs PC-20
Amaco PC-20 vs Ravenscrag Floating blue

Links

Materials CMC Gum
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.
Articles 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.
Articles 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.
URLs https://docs.google.com/spreadsheets/d/1XLPlIrW4tXnqAyowrE_P96OmbMfzEA-ggkYHcqBHtSU/edit?usp=sharing
CMC Gum Calculator - Worksheet
Glossary Glaze 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.
Glossary Dipping Glaze
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.
Glossary 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.
Glossary Limit Recipe
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.
Glossary Spray Glazing
In ceramic industry glazes are often sprayed, especially in sanitary ware. The technique is important.
Glossary Glaze Layering
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?
By Tony Hansen
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