Schott S-8 Encasement Gobs

[brads]

I just stumbled across this thread. Mike, if your gobs are from the late '80's or early 90's they aren't from Schott's last melt of S-8, which was somewhere in the 2000s, but might be from their first one - the first that was released to the public, at least. Depending on who you do/don't believe, S-8 is a close lookalike to the glass that Paul Stankard contracted Schott to melt for him in the late 1970s. (Yeah, I know it should be "whom" you believe, but it just sounds weird.) I seem to recall that the first melt of S-8 they sold to the public was done around 1988 or '89. Prior to that, lampwork weight makers used Schott S-5, which has an expansion closer to the German color bar that furnace workers commonly use. I know that because I bought a pile of S-5 from Schott's last melt of S-5 in the mid/late '80s. After that they switched to melting S-8 exclusively (when melting glass for paperweight makers every few years). Some paperweight makers were not happy about that, and over the years I've sold off most of my hoard of S-5 to several of them.

Here's some info that may be of interest to people regarding S-8. The measured CTE (expansion) of S-8 is 109 from 20-300C, which would make Mike's stated CTE of 108 about right for a measured range of 0-300C. As he also said, there is no lead in S-8. There is some barium and there is some boron in it, along with the usual soft glass ingredients. But there really isn't anything magical about it. The composition is just very well thought out - and the glass skillfully melted in a well designed glass furnace by people who really know what they're doing, and aren't skimping on quality.

Information about the glass that some may have seen posted by a self-proclaimed "expert" elsewhere is largely bullshit. It doesn't contain a boatload of barium and it isn't a lead glass. (Both are claims that I've seen this particular charlatan make at various times.) I know this because I happen to know the composition of S-8. And no, I'm not going to share it with you. Feel free to whine about how unfair that is...

The primary reason S-8 "fits" 92-96 COE furnace glass is, well... it actually doesn't. But correctly done, the way a paperweight is constructed will keep it from cracking. Not surprisingly, most "104 COE" glass is a better fit for S-8.

Below is the data sheet for S-8. Note the date on it. If my memory is correct, and after 25+ years it is admittedly a little foggy, this was the second melt of S-8 that Schott made available to the public. The first one was a couple of years earlier. Prior to that, their melts for paperweight gobs (proper name: Art Encapsulation Glass) were S-5. I recently stumbled across my original invoice for the S-5 I purchased from their last melt of it. But, true to form, I've misplaced the invoice again. (I put it in the proverbial "safe place" so I wouldn't lose it.) If I manage to find it I can give an accurate date for the last S-5 melt and better determine when the first melt of S-8 happened.