Is there a devit fix?

[SWalsh]

If you remelt a project suffering from devit, do you get a second chance to change anneal schedule to avoid devit on the project? Tried a drop ring today but dropped through the mold and I'm not sure but it may be devitrification on the top of it. When I found it died on me, I shut the kiln off and vented the kiln about an inch. I let the whole thing cool... rather than anneal. It looks rather wrinkled as it actually fell through and slumped over on itself. Not sure if the "wrinkles" are wrinkles or devit. What are your thoughts? And thanks, in advance. Sue

[SWalsh]

Nevermind! I found my own answer in one of the old posts in the archive. Thanks, Cynthia! Sue

Posted: Mon Jul 28, 2003 2:08 pm Post subject:

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Okay, this quote is long and technical, but it's what devitrification is. For the most part it is talking about hot glass, but goes on to discuss kiln worked glass as well.

From "Dictionary of Glass: Materials and Techniques" by Charles Bray

Devitrification. Glass is a material with a randomly distributed structure which, on cooling from a molten state, tries to regain a more regular crystalline lattice. It is restricted from doing so because of the viscosity of the material. If, however, the glass is held at its liquidus temperature for any length of time, the network modifiers can help to form crystals, of, for instance, devitrite. These localised crystals have a changed composition from that of the glass resulting in a different coefficient of expansion. This creates areas of strain and tends to promote cracks. For those responsiblie for large scale production, devitrification is something of a nightmare and as far as is reasonably possible, both glass composition and the coolong rates are designed to aviod it happening. Because of this, the normal procedure is to cool the glass as quickly as possible to a temperature at which it can be annealed.

A glass that has a high lime content tends to devitrify easily. An increase in the lime content also reduces the working time so a manufacturere who wishes to produce a glass which hardens quickly to suit his forming process may decide to increase the amount of calcia in the batch. If he overdoes this he increases the risk of producing crystals of devitrite. Devitrification can occur in the furnace, particularly if there are parts of the tank or pot which remain cool. this can cause the production of devitrification stones and/or cords and create a subsequent loss of material.

The danger of devitrification in studio glass is exactly the same as in the factory. It often occurs in kiln-cast glass where the cooling of the material in a mould tends to be slow. If the result is one in which there is uniform transulcency or opacity then this may well be caused by some other factor such as glass to glass phase separation or silmply the effect of dust on the pieces being fused or cast. The studio glassmaker has always taken a rather wide, and often incorrect, interpretation of devitrification and anything which in the process of creating cast or kiln-formed work has become opaque or traslucent tends to be labelled as being affected by this. Experience would suggest that anything which has developed individual patches of what could possibly be crystalline material is likely to be suspect and should be discarded whilst glass which has become evenly opaque or transucent throuthout will probably be safe provided that there has been adequate annealing and if all the pieces of glass involved are compatible.

Studio glassmakers producing kiln-formed pieces need to be particularly careful when trying to re-melt or fuse to gether glass which contains areas of devitrification with glass which is clear (meaning glass that is doesen't contain areas of devitrification). The devitrified areas of glass will not melt as readily as the normal glass and as a result will not move into the required form. Even if the two materials become incorporated to any extent the result will inevitably be unstable an will probalby crack or simply fall into pieces.

This definition of devitrification doesn't give an answer to "why devit on the upside but not the kiln side?", but might offer some groundwork to make some suppositions from.



_________________
Cynthia

[Judi Charlson]

I just took three box cast reverse relief pieces out of the kiln. They turned out nicely, except there was some clouding and "pruning " around the edges of two. Plus a few"fins"on the edges. I plan to just cold work the edges and clarify the rest with Watco oil or something. I followed the directions from the WG confersnce workshop, except that I increased time to melt from 1 to 21/2 hours and increqased annealing time from3 to 10 hrs, ( I used 2 1/2 billets instead of two because my relifs were deeper.I hand anouther larger plaster cast in thew kiln too)

My question,Was the melt too long?@ 1600. Should it have been shorter and higher(1650). Should I have crashed the kiln to lower temp to 960 faster?
If anyone has used this box method and has suggestions about devit, pruning and fins, please respond.
Thanks
Judi

[Brian and Jenny Blanthorn]

If you remelt a project suffering from devit, do you get a second chance to change anneal schedule to avoid devit on the project? Tried a drop ring today but dropped through the mold and I'm not sure but it may be devitrification on the top of it. When I found it died on me, I shut the kiln off and vented the kiln about an inch. I let the whole thing cool... rather than anneal. It looks rather wrinkled as it actually fell through and slumped over on itself. Not sure if the "wrinkles" are wrinkles or devit. What are your thoughts? And thanks, in advance. Sue

I recently posted some stuff on devit

Brian

In addition 2 what Charles wrote

Devit grows from solid stuff like dust, grit, scratches etc

Devit will not grow from liquid stuff like normal glass

Anti devit stuff works by disolving the solid stuff / reducing the liquidity of the glass so the devit cant grow

Brian