When visited in our last blog episode, Ed the Cat was sitting prettily – a word he would surely have detested – in his Chintz-y metal box on our kitchen windowsill. Dan still hasn’t managed to come up with a bone ash opal formula that’s worthy of Ed’s six ounces of dust.
What’s the hold up? It’s a color formulation problem. Ed just can’t become anything other than a red, yellow or orange glass – colors that are typically made with cadium/selenium oxides. Those of you who know a little glass chemistry will recognize them as the most ornery, irascible and unpredictable colors in any glassmaker’s palette.
Dignified? What’s dignified about sitting around in a tin can waiting for a glass chemist to get his act together?!
Cadmium/seleniums are so ornery, in fact, that I was pretty surprised when Uroboros’s Bill Ward recently appeared on the Warm Glass bulletin board to state that “…our 90 [COE glass] was designed to be compatible with Bullseye Tested Compatible and these have been comingled successful by many artists since 1989.”[sic]
Checking out the Uroboros website, I saw the same claim “Products at the 90 COE expansion point are tested compatible with Bullseye glass”
Over the years I’d heard about problems with mixing this so-called “90 COE” glass with ours, but I’d never seen any formal testing to investigate the issue. Bill’s remark got me into gear. (Or maybe it was Ed’s peevish spirit).
Either way, Dan drove on over to our local glass retailer and picked up samples of all the Uroboros red, yellow and orange glasses that he could find on the shelves.
TRIPLE FIRES: Measuring Irascible Glass
Because cad/sels have a tendency to change their internal composition on multiple firings – as might happen with a color-bar/full fuse/slump series of firings – we expanded our testing of these glasses years ago to include what we call Triple Fires.
In a Triple Fire test the same sample of glass is fired three consecutive times to a full fuse temperature. After each firing, the test sample is cooled to room temperature and viewed for stress at the interface between the chip and the standardized base glass. If, after any one of the three firings, the chip shows stress greater than our accepted deviation of 2 degrees low to 4 degrees high, it is considered incompatible with that standard and its related glass will not be labeled Bullseye Compatible.
After we instituted Triple Fire Testing both Uroboros and Spectrum glass factories followed suit with claims of “three meticulous test firings…measured for ….COE-shift”.
But – as I’ve been ranting all these weeks – unless a glassmaker explains exactly what their testing protocol is, I’d consider the claims to be just…well…marketing.
READING THE TRIPLE FIRE TEST
I hope you’re not totally bored after all these weeks of looking at fuse test bars. Because the ones coming up here are instructive.
Of the six Uroboros samples that we purchased randomly from our local glass store, four failed our triple fire testing.
This is what they looked like through the polarimeter.
First, the good news (for users):
UB’s 60-606.90 Grenadine Red Transparent passed the test. Against Bullseye’s base clear standard it showed to be within the acceptable range with 1/2 degree high on the first firing, 1 degree high on the second and 1/2 degree high again on the third.
The 60-357-90 Lemon Chiffon Yellow Transparent also passed with 1/2 degree high of measurable stress on the first firing, 1.5 degrees high on the second, and 1.5 degrees high again on the third.
Then our cadmium/selenium friends started to unravel.
The 60-608-90 Tangerine Transparent was just outside our standard range on the first firing at 5 degrees high. Then it went further out to 6 degrees high. On the third firing it went back just slightly lower at 4 degrees high.
Note that most glasses, when their chemistry alters due to more firings, will trend consistently in one direction – either going increasingly higher or increasingly lower. A glass that behaves erratically like this one usually indicates inhomogeneity within the piece of glass. But without access to the full run of glass from which this sample was taken, it’s really not possible to say for certain why it behaved this way.
The 60-356-90 Lemon Yellow Opal was compatible to our standards on the first firing at 1 degree high. On the second firing it went over the limit at 7 degrees high. Then it shot even further out to 11 high on the third firing.
The 60-351-90 Marigold Opal was a loss right out of the starting gate: 6 high, then 10 high, then 15 high.
The 60-604-90 Red Opal was the biggest problem in the bunch. It came closer to matching a “96” glass than a “90” with headlights flaring at 10 high, 15 high and finally 20 high.
Of course I’m very curious to know what Uroboros uses as their testing protocol, but since Bill wouldn’t answer the question at Warm Glass, we’ve had to try to figure it out for ourselves.
As I stated on Warm Glass, I’d certainly like a dialogue between factories that would help users really understand what compatibility in our field means. But I like my discussions in public. Until we can have them here, I guess I’ll just drone on alone.
Meanwhile, we’ve left Ed to muse on the prospect of his precarious afterlife as a cadmium/selenium jewel.
In the end, a stable cad/sel is a delicate work of art, not to be taken lightly or glossed over with vacant claims of “COE 90″ or “…the industry’s most rigorous testing”. Sometimes compatibility just isn’t so easy. But neither was Ed – and we loved him for it.