WarmGlass.com

We just ran some strip tests on various glasses. The devit shows on the top side but even the worst cases were shiny on the shelf/paper side. Can someone explain the mechanics of what's happening here on the shelf side? It's a standard shelf with the new BE thinfire between it and the glass. I've read about Borax and I've used super spray etc. It's just funny that it doesn't devit on the down side? How come?

Thanks,
TrishV

TrishV wrote:We just ran some strip tests on various glasses. The devit shows on the top side but even the worst cases were shiny on the shelf/paper side. Can someone explain the mechanics of what's happening here on the shelf side? It's a standard shelf with the new BE thinfire between it and the glass. I've read about Borax and I've used super spray etc. It's just funny that it doesn't devit on the down side? How come?

Thanks,
TrishV

air contact causing crystallization of the top surface? cooler temps due to heat shielding of the shelf?

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.

Thanks Cynthia, this is very interesting
TrishV