Shp123 Posted March 30, 2020 Report Share Posted March 30, 2020 Hi there! I'm having a bit of trouble using my amaco underglazes on porcelain. When I fire with clear glaze the greens and pinks fade drastically go a bit brown. I've seen some lovely vibrant work and I was just wondering how to achieve this? Below about 1235°C my glaze gets a bit bubbly. Any help would be greatly appreciated!! Link to comment Share on other sites More sharing options...
Bill Kielb Posted March 30, 2020 Report Share Posted March 30, 2020 1235 C is cone six’ish so below that I guess it does not melt? Here is what I can say for sure: not all colors remain vibrant, some burnout more than others as temperature goes up. Some clear glazes also affect the color of the underglaze. Most folks create test palettes with their clear glaze over half the sample colors just so they know what the color change will be. From there they will try different clear glazes and even different manufacture’s underglaze to find the most stable shades for themselves. One final issue would be some underglazes (colors) act more refractory than others and not all clear glazes melt well over them. So more testing to figure out the most refractory colors with a particular overglaze. So I think the answer to your question is testing, pick a clear glaze that you have success with and make test palettes of the colors you like fired to various temperature ranges so you know what combination fades and what doesn’t melt well all on the clay that you will be using. Early on it was thought that zinc free clear was the simple solution, but it truly is more than that, although clear glazes made with zinc will wash out some colors more so. Testing is the only way to be reasonably certain. Link to comment Share on other sites More sharing options...
Shp123 Posted March 30, 2020 Author Report Share Posted March 30, 2020 Thank you Bill, that’s very helpful! Link to comment Share on other sites More sharing options...
liambesaw Posted March 30, 2020 Report Share Posted March 30, 2020 Zinc turns anything with chrome (that's pinks, lavendars and greens) to a washed out brown or tan. It also happens to be a really popular additive for clear commercial glazes. We ran into this problem at school because we had two amaco clear glazes, one said HF and one said ZF, and people chose the HF one because it was "high fire" and didn't choose the ZF because they thought maybe it was zero fire or something. Anyway, that was my personal story with zinc and underglaze. Also at higher temperature chrome volatilizes, but I wouldn't expect that at cone 6. Link to comment Share on other sites More sharing options...
Min Posted March 30, 2020 Report Share Posted March 30, 2020 As well as the zinc issue the amount of calcium in the glaze can make a difference in how pinks/reds/purples turn out. If those underglazes use a chrome/tin stain then the covering glaze needs to be high in calcium or the colour will be washed out. If the pinks/reds/purples use a cadmium inclusion stain then the covering glaze isn't as fussy as to it's makeup. Link to comment Share on other sites More sharing options...
neilestrick Posted March 30, 2020 Report Share Posted March 30, 2020 I've had a lot more problems with Amaco underglazes than with other brands. The Speedball and Mayco underglazes hold color very well at cone 6 with several clear glazes I've tested. Many of the Amaco colors change or fade with glazes that work fine with the other brands. Either keep testing different glazes or start testing other brands of underglazes. Link to comment Share on other sites More sharing options...
Min Posted March 30, 2020 Report Share Posted March 30, 2020 It would be really helpful if the companies that make underglazes say what the base glaze should or shouldn't contain like Mason stains does. Spectrum lists 561,562,563, 567,568 and 569 as containing cadmium. I contacted Speedball a few years back and asked which of their underglazes contained cadmium and was told their orange, yellow orange, melon, red, burgundy, violet, pink and purple all do. Link to comment Share on other sites More sharing options...
Shp123 Posted March 30, 2020 Author Report Share Posted March 30, 2020 Thank you Liambesaw, Min and Neilestrick! I'm going to take a good look at the constituents of my glaze and I might change to Speedball underglaze. Thanks again! Link to comment Share on other sites More sharing options...
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