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Control gel using Veegum, brushing properties with CMC gum
This is G1214Z1 brushing glaze (with 5% titanium added). For a 340g powder batch (to get a pint) my initial target is 5g CMC gum and 5g Veegum. CMC controls drying speed and Veegum the amount of gelling. I first mix the CMC with the powder and shake the whole batch in a plastic bag. Then I add it all to 440g of water in the blender jar and mix it well (making sure no agglomerates remain (stage 1). Stage 2 is adding the VeeGum slowly, while high-speed blender-mixing, this is important because it enables tuning the degree of gel (which cannot be predicted). Because this recipe has little clay, it took all 5g of Veegum without overgelling (the entire mass moved freely in the mixer jar). But it did gel overnight (so 4g would be better next time). By contrast, it is the brushing behavior that demonstrates whether the amount of CMC is right. Not enough and coats dry too fast and go on too thick. Too much and it dries too slowly and too many coats are needed.
Related Pictures
Paint another layer onto a fired glaze? Yes. With CMC gum.
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The cone 6 mug on the left has the G3933A glaze, applied as a dipping glaze. It turned out poorly - crawling from corners and looking thin and washed out. I made a brushing glaze version of this (which adds 1.5% CMC gum), I keep it around for this very purpose. It has a high specific gravity (unlike commercial ones that have high water contents - they will run and go on too thin if you try this). Because of the gum it dries hard, there is no shrinkage or cracking. On a second firing, using the C6DHSC schedule again, (mug on the right) the surface is transformed - thicker, more vibrant color (being picked up from the underlying body).
What should the consistency of CMC gum solution be?
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This is CMC gum 35g/liter gum solution after it has been thoroughly hot-mixed and cooled to about 30C. As it cools further and sits it thins. Gum solutions can have a higher CMC content, up to double this, but they are more difficult to use.
Most often, gum solution is intended to augment the water when batching a recipe - not adding to a glaze slurry. This is because adding enough to have any effect on an already-mixed glaze thins it (so an equal amount or more water should first be removed). For example, consider converting a dipping glaze to a brushing glaze: Adding the equivalent of 1.5% CMC gum powder, via this gum solution, to a gallon of glaze would also add almost a liter of water. Better to blender mix in the CMC power. CMC gum is often used in consort with a gelling agent like VeeGum, enabling more water (this is done in many commercial bottled glazes).
Here is my setup to make brushing glazes and underglazes by-the-jar
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Let's make a low SG version of G2934BL, totally DIY. Weigh out a 340g batch of dipping glaze powder. Include 5g Veegum (to gel the slurry to enable more than normal water) and 5g CMC gum (to slow drying and impart brushing properties). Measure 440g of water initially (adjusting later if needed). Shake-mix all the powder in a plastic bag. Pour it into the water, which is blender mixing on low speed, and finish with 20 seconds on high speed. This just fills a 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel, the CMC for slower or faster drying and the water amount for thicker or thinner painted layers. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Dipping glaze recipes can and do respond differently to the gums. Those having little clay content work well (e.g. reactive and crystalline glazes). If bentonite is present it is often best to leave it out. Recipes having high percentages of ball clay or kaolin might work best with less Veegum. Keeping good notes (with pictures) is essential to reach the objective here: Good brushing properties. We always use code-numbering (in our group account at Insight-live.com) and write those on the jars and test pieces. This is so worthwhile doing that I make quality custom labels for each jar!
Veegum CER Saturated suspension
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This is the most viscous suspension we can make using hot water and a propeller mixer - 300ml water with 13.25g Veegum CER. In glazes having low clay content adding this gels the slurry, enabling higher water content to improve its brushing properties and slow down drying. In slurries having plenty of clay pure CMC gum is usually better (unless a high-water-content slurry is needed). Of course, keep in mind that anything with CMC gum can be subject to microbial attack.
Testing the water content of a ceramic glaze
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In this case I need to know the water content of a glaze to know how many grams of CMC gum to add to achieve a 0.5% addition (to improve brushing properties). I put a teaspoon of the glaze (well stirred) on the tray. It weighed 5.04g (this 0.01g scale is accurate to 1/100th of a gram). Then I dried it under a heat lamp and now it weighs 2.46g. The water content of the slurry is thus (5.04 - 2.46) / 5.04 * 100 = 51.2%. That means the solids content is 100 - 51.2 = 48.8%. I want to blender mix 0.5% CMC into this brushing glaze (because it is drying too fast). The jar with glaze weighs 680g and the jar itself weighs 35g. Thus 680-35=645. 48.8% of 645 is 315g of powder. 0.5% of 315 is 315 * .005 = 1.6g of CMC gum.
Videos
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. |
| Materials |
Veegum
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling agent in slurries. |
| Glossary |
Micro Organisms
Ceramic glazes and clay bodies can host micro organisms. They can be just a nuisance, a source of worry or can render a product useless. What should you do? |
| Glossary |
Rheology
In ceramics, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers). |
| Glossary |
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. |
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