Thread: cullet shortage
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Old 01-30-2018, 08:23 PM
Dave Bross Dave Bross is offline
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Basics of Batching Clear

Very basic - sand, soda, lime glass oxidised clear (so it will fine)

You want:

As close to 70% sand as you can get, usually in the high sixties due to necessary compromises.
As for sand, see if you can find Short Mountain silica. It works better than most. Usually found at pottery suppliers, just like everything else you'll need here.

At least 8% modifiers, usually lime because it's inexpensive but could be almost anything in the 2nd column of the periodic table.

Try to keep it under 18% total alkali (see 1st column of periodic table for alkalis), usually sodium for economy but it won't polish well or be as pretty without some potassium. If you have to go over 18% alkali (matching Moretti?) you can still do it without devit if you use...

Enough alumina to keep it durable. Nick Labino's rule for all soda lime glass...divide total alkalai by 8 and that's your alumina percentage. This applies to all glass. Go below this and eventually the glass will devit.

Add 0.2% antimony and 0.4% sodium or potassium nitrate so it will fine out.

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OK, now let's spend a little more money and get some way nicer glass.


Again, potassium. Yeah, it's a bit expensive but 3-5 % will make a noticeable difference in appearance and polishing. % 5-6 will give you a glass you can use as a color base with things like manganese purple and not have the color brown out.

Strontium is inexpensive and works well replacing around half the lime. It does nice things for the appearance of the glass. Very similar to what lead does but it's not toxic.

Zinc will brighten up even your basic cheap-o soda lime alkali only glass at around 2% with a big bonus of much added durability.

Lithium will make it melt like gangbusters even at very small percentages. 0.2% is enough to get the basic benefits, as in speeds melting, improves glass strength and reduces thermal shocking. The price is spiraling up daily but worth whatever you have to pay if you're melting in something under powered or want the benefits and an easy way to change working characteristics.

Of course, if you change things, you'll have to tinker with the amounts of other things to keep it compatible. See below about spreadsheets.


Want to change the working characteristics?


Lithium drops the viscosity and extends the working time of the glass.

Leave it at 0.2% or cut it out altogether if you prefer shorter and stiffer glass to work.

Want sloppy and long like SP? It has roughly 0.9 to 1% lithium so take yours up in that range.

Need to go the other way on that or just want it much stiffer? Add more alumina in tiny percentages. A little goes a long way.



To match expansions you'll want to track what you're doing in a spreadsheet. You're welcome to a copy of mine, it's open source and free. Email me because I don't think there's a way to post a working version here.

Changing the amount of sand can be one of the easiest ways to match an expansion number because one pound of sand in a 100 pound formula will move the expansion about a half a point. In my particular case it moves the expansion .6 per pound added or subtracted.


All the calculations in the world only give you a "ballpark" number for glass melts. The truth is in melting it and seeing what you get.

There are too many ever-changing variables to get a truly accurate number on paper.

If the first melt missed, then you go back to your E&T calculations to get a ballpark number of what you'll need to add on the next melt to raise/lower expansion.
Then you melt that and see what you have.

This is where the E&T numbers in a spreadsheet really shine, they seem to be even more accurate for this "adjusting" phase.
Sometimes it takes a third melt to dial it in where you want it, but rarely more than three unless something else among the variables has gone wrong and is messing with you. Stuff like thermocouples, melt times/temps., hyroscopic moisture in the chemicals, etc. etc.

I always assume I'm in for three melts to dial something in and I'm often pleasantly surprised when I hit it in two.


The more you can standardize your process and sources of supply the fewer strange surprises you'll have.

Water in your batch chemicals can change quantity with the weather and screw up your results.

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Be careful to balance thought with empirical experience...or in simpler terms don't let too much analysis get in the way of hands on experience.

This is particularly important with glass formulas/melting because everyone gets unique results depending on their location, equipment type, phase of the moon (kidding) etc.

Something I learned the hard way myself...

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I mix batch in a bucket or small plastic drum cut in half. I use a drywall mud or paint mixer and a drill, cardboard with a hole for the shaft on top of the bucket to keep the dust down. Fast and some of the most complete mixing so far. If you want super-sanitation on dust issues the industrial supply houses sell plastic bucket caps that look like a womens showercap. You'll have to add the hole for the shaft. I mix outside so the dust just fertilizes the grass.

When I was mixing a lot of clear I bought an inexpensive Harbor Freight concrete mixer.

Use a respirator and rubber gloves at a minimum for your protection.

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I know, you want an actual formula. I'll include some screen shots of my spreadsheets for the basic and something fancier with zinc and strontium.
Attached Images
File Type: jpg Basic Clear.jpg (69.0 KB, 45 views)
File Type: jpg Basic Clear Fancy.jpg (69.9 KB, 31 views)
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