|Monthly Tech-Tip |
Potters are used to making dipping glazes that they weigh out and mix from recipes. Hobbyists commonly use bottled commercial brushing glazes. Did you know that a dipping glaze can be turned into a brushing glaze by the addition of Veegum (or Veegum CER) and water? Do you know what a base-coat dipping glaze is? Here is a quick overview: Dipping glazes need to go on to bisque ware evenly, be thixotropic enough to hold on at thickness and drain and dry quickly. But they don't need to dry hard. Brushing glazes need a cohesive slurry that dries slowly and hardens well on drying. They also must adhere to the body really well so that multiple layers can be applied (individual layers go on thin). Base-coat dipping glazes are in between, they need to dry fast enough and gel well enough to make application by dipping possible (although less practical) and they need to adhere well enough to tolerate another layer without letter go of the body.
These work well from cone 04 up, they are better than any commercial transparent brushing glaze we have used. And far better than glazes made using raw sources of boron (like ulexite, colemanite, Gerstley Borate). These glazes have lower thermal expansion and do not craze on any body we have tried (yet are ultra gloss and ultra clear). They are G1916QL1 and G3879C. We developed them for use on the dolomite-based (rather than talc-based) L4410L low temperature art clay body. These are a good demonstration of the technical and economic sense it makes to use highly fritted glazes at low temperatures. Having a good base glaze is the key to adopting low temperatures for your production. You would likely agree that no stoneware glaze has melting patterns like these shown in this melt fluidity test! These recipes and all details about their development and adjustment are openly available.
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!
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.
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.
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?