Blending an engobe and a glaze to produce an underglaze

These fired samples demostrarte how you could develop your own Stroke & Coat glazes. I made these blends by dewatering an engobe (L3954B) and glaze (G2926B) and then hand-mixing line blending them. The left column is the fired blends (cone 6), the right one the unfired ones (showing the increase in soluble salts as the percentage of glaze increases). The right one shows the increase in melt fluidity. The top ones are 30:70 glaze:engobe and the bottom ones are 90:10 glaze:engobe. Notice the big change in melt fluidity between 60 and 70% glaze (third and fourth from bottom). And that the 70% one is quite glossy even though it has very low melt fluidity. Somewhere between 60 and 70 we could isolate a glaze-like product having underglaze stability. Stains could be incorporated and the proportion of glaze:underglaze adjusted according to its effect on melt fluidity. Doing this yourself enables targeting an exact melt fluidity for each color at the target temperature. The commercial product does not do that. You can also use more stain then they do to get more vibrant color for thinner applications.

Related Pictures

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

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!

Stroke & Coat colors

This custom sample board was made by Solange Roy, it is great evidence of her meticulous and thorough approach to ceramics. This picture is taken at an angle to show the surface finish and character. These are Stroke & Coat ® underglazes, they are made by Mayco and widely used and praised. Although used as such by countless potters, they are not traditional underglazes (like Mayco Fundamentals or Amaco Velvets). They are heavily pigmented and have a highly controlled viscous melt fluidity. Applied by brush they give watercolour-like effects in thin applications and increasingly opaque coverage with each added layer. These products give evidence of meticulous lab work (like this board done by Solange), technicians would have had to do melt fluidity tests of each color and fine-tune the base recipe to get the desired degree of melt fluidity (compensating for the unique effect on melting of each stain type and percentage needed).

Links

URLs	https://www.maycocolors.com/color/fired/stroke-coat/ Stroke & Coat® are glazes, not underglazes. They are heavily pigmented and have a viscous melt. Applied by brush they give watercolour-like effects in thin applications and increasingly opaque coverage with each added layer.
Glossary	Underglaze An intensely pigmented highly opaque non-melting ceramic material mix meant to adhere best to leather hard pottery and fire-fit the body. Often transparently overglazed. Starter recipes.

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