Author Topic: An attempt at a SP87 like glass batch  (Read 3123 times)


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An attempt at a SP87 like glass batch
« on: January 27, 2019, 11:59:13 AM »
Corning performed an analysis of Spruce Pine that has been published in Frank Wooley's book, Glass Technology for the Studio:

Measured expansion: 94.9

SiO2 - 72
Al2O3 - 1.8
Na2O - 14.9
K2O - 0.4
Li2O - 1.0
CaO - 7.9
MgO - 0.2
BaO - 0.5
ZnO - 1.0
Sb203 - 0.2
F - 0.1

I thought it would be fun to use a batch calculator to try and dial in a formula based on this analysis. Recall, my calculator has a price/lb feature that is pretty revealing and sometimes a go, no-go answer.

I've uploaded this recipe in the calculator so you can easily play with the numbers. Here's a screenshot:

So, before we go and run out to make SP87, look at that price per pound at $0.94!  :o
Bulk buying power is the difference especially for all that lithium. This is about what I'm paying from OCR before shipping. I recently got the "snowflake" version which doesn't really require a special designation. It's simply non-pelleted, powder batch. Better than SP bricks. It did come up 12 lb bags which is kind of nice, but I don't throw whole bags in my electric furnace.

I need to add some potassium contribution from potassium nitrate as well. The SP color base formula has some discussion on Craftweb about what it has removed. I believe it was nitrates and antimony but I'll check back and comment later.

You may notice that the MgO % is pretty far off. I'd explain this by pointing to the one MgO source from typical batch...silica. Depending on your sand source you're going to have more or less MgO contribution.

The "Other expansion" is close to the measured at 93.9...about a point off. I could hone in on that number by tweaking ingredients but you might be better of making a test batch and seeing where it lies. Or just buy SP87 from the pros. ;)

« Last Edit: January 27, 2019, 01:08:59 PM by Administrator »


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Re: An attempt at a SP87 like glass batch
« Reply #1 on: February 02, 2019, 12:01:13 PM »
I was asked about the SP87 recipe and the color base. A Craftweb post reminded me of a clue for its ingredients.
In a discussion about coloring SP color base, it was noted that soda is often added back to the base. Kenny Pieper pointed out that the color base has the following modifications to SP87 clear:

- no antimony
- the subtraction of 1.7lb. sodium nitrate from 98lb. of batch.

With this is mind, I will make a new version of the SP87 theoretical formula with sodium nitrate.  :D

The rest of this post including the Penland list of SP base color additives is in the similarly titled post here.
« Last Edit: February 02, 2019, 12:03:16 PM by Administrator »