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blschmoyer

Velvet Underglaze Firing Schedule

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Are you applying a glaze over these underglazes at all?

 

I've never had the velvet underglazes affected by a firing schedule at all. Generally, if left unglazed at ^6, they are a dull matt version of the color they're supposed to be. They don't really represent "true" colors without a clear glaze over them of some sort.

 

As for color alteration (like your blue -> purple), temperatures can alter the color in my experience (reds fade or orange, blues turn purplish, etc).

 

Another factor is clay body. On a buff stoneware, the colors will look VERY different than on a porcelain (far more muted / muddy).

 on the buff clay

 

I think we need some more information on what you're doing before we can definitively ascribe a problem.

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velvets are all incredibly stable in oxidation at cone six, 

 

I respectfully disagree. Some of the colors will shift at cone 6, especially depending on the clear glaze you put over them. Amaco makes a chart showing the differences because of this, HERE. Velvets are formulated for low fire, however many work well at cone 6 and above. The shift is not due due to the clay, but rather that the stains and colorants they use are not necessarily stable at temperatures above low fire. This is especially true for warm tones like your purple. But I have seen much more color shift than what they show on their chart. Test, test, test. I agree with Norm, your firing schedule shouldn't matter. Underglazes do not go into melt like glazes, so the firing schedule has far less effect on them. Firing in reduction will change them even more than shown on the chart.

blschmoyer and Norm Stuart like this

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I can confirm, what Neil stated.

In one of my classrooms, we fired cone 6 stoneware. Generally we applied underglaze before the bisque firing. The colors came out well then. However, once they were clear glazed and fired up to full temperature, some of the colors darkened or changed.

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I use Amaco LUGs extensively. If you are having major color shifts look at your clear glaze once I switched to Amaco Clear ZINC-FREE my colors became much more stable. I have very little color shifts now and only notice the reds and purples look a bit burned out when I fire them to ^6 without the clear glaze over them. All of the colors will darken and appear brighter once they are clear glazed and fired. You learn to compensate for this when you apply them as you become more familiar with them and it becomes 2nd nature to adjust your shades for the final outcome. I hope this helps.

 

Terry

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