Vitrification on Opal?

[rpastore]

I bought an Easy Fuse COE 82 sample set from Youghegheny Glass http://www.youghioghenyglass.com/sampleinfo.html

I did a test firing and fused 45 1" by 1" pieces on float glass that Yough sells using their recommended firing schedule at http://www.youghioghenyglass.com/easyfu ... hedule.htm with a max at 1560 F.

Every single opal piece had vitrification on the upper surface. The other pieces were fine. I am not sure what caused this but if anyone has any experience or ideas why the opals were singled out, I would appreciate any insights.

[Bert Weiss]

First it is important to understand the terms and what they mean. What happened is devitrification. Vitrified means, in a glassy state. The definition of glass is a non-crystalline solid. When crystals form on glass is is called devitrification.

I want to explain opal glasses but I don't really understand them well enough. They are a short step away from crystalline. These glasses are easier to devitrify.

My advice is to fire the glass to 1480ºF and hold for 20 minutes. In my opinion 1560 is too hot. I have some experience working with Yough transparents, and they behave well for me. I've never used their opals beyond tests with all the samples.

If you have to fire in to the devit range, you need an overglaze, either borax or spray A.

[DonMcClennen]

One of the common causes of devit is "glass not squeakey clean"!!! ...Make sure you glass is CLEAN...CLEAN...CLEAN. Don't use chemicals just some diluted dish detergent then RINSE...RINSE...RINSE. This is a simple solution to 90% of devit. problems.
Don

[Barry Kaiser]

I thought I would put this in this thread. In your case, it sounds like you are just cutting and it really doesn't apply in that case, but here it is anyway.

There is a huge difference between "devit" on the top (flat) surface, and the "devit" on the sides of the piece occurring after grinding. I refer to the side devit as the gray scum of death.

The top devit is the glass itself devitrifying. Most prevalent cause is improper cleaning. Can be the cleaning agent is contaminated with something that enhances the devit process. Can be that the glass is just prone to devit (though that is not usually the case with the enhanced glass formulations these days).

The side devit is more complicated and probably more prevalent. Improper cleaning of the edges is a major problem. This is why some people put the ground pieces in water to prevent the grinding swarf from staying on the glass. That swarf enhancing the process of enhances the probability of devit.
Second, not grinding the edge to a smooth enough finish really is probably the cause of most of the edge scum. We go to a minimum of 400 grit for our edge if we are not going to further process the edge. Even then we use a borax based devit agent on the edges. We have a tutorial (see below) on how to simply and very inexpensively make a borax based devit agent.