| Original Message: Re: Colorant Failure in chevron brick red glass- Perhaps this will be more helpful Jamey |
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The reason the red (hematinone ) glass is red to the eye is that at a microscopic level there are a bajillion tiny crystals of red copper oxide suspended in the glass. These crystals grow in a condition which is akin to torture for the glass where it its held at a relatively low temperature with a super saturated copper content in an oxygen starved atmosphere in the furnace. A variation of this process is used to make aventurine. The growth of these crystals takes time. Aventurine glass will likewise appear green if the crystals are cooked out of it. so care must be taken to keep it from becoming too hot in design application after the crystal growth has been done. The green in the glass is a function of the red copper oxide crystals going non crystaline and reverting to copper green. It would provide identical chemical analysis if a spectrographic analysis were done. The red glass in question would rather be green, but is red to the eye due to the red copper oxide crystals being present in mega numbers. The crystals are flat plate like structures which align themselves much as a pile of paper would if thrown on a table. All those tiny flat plate like crystals pile up and create a red opaque look to the glass. If you slice any of that brick red copper glass thin enough it will appear green. Many of the older chevrons have green striations in the red layers giving rise to much discussion over how many layers are in the bead. These questions arise with fair frequency on this forum. Without question when you see those dark striations associated with the brick red it is a function of the red degenerating from its suspended red copper oxide crystal state to a more relaxed copper green. The bead shown in this thread presents an interesting view into this phenomenon. With modern industrial methods giving greater control to temperature and atmosphere, modern glass makers have been able to keep better control of this type of red so that one sees this effect rarely in the newer beads.
The book Colored Glasses by Woldemar Weyl published by the Society for Glass Researchers in England can provide much information on glass colorants at a very scientific level. It's a fairly obscure book printed in 1951 and is long out of print.
Hope that this clears some of this up for you.
best,
art
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