Switching Glass
You’re not stuck.
The most common reason studios stay on a glass they’ve outgrown is a myth: that you can only switch when you’re putting in a new pot or liner. You don’t have to wait. Switching is really just dilution — here’s how it actually works.
Switching glass is a dilution problem — not a compatibility problem.
The thing we hear most is “I’ll switch when I repot.” It’s understandable — an empty furnace is the cleanest moment to change — but as a requirement it’s overblown. You don’t need a new pot or liner to change your glass. A reline is just a convenient time to do it, not a precondition.
Here’s why: you’re not protecting against two glasses that fight each other. As long as you’re moving between similar soft glasses, the new batch is close enough to the old that they live together fine. The only thing you’re actually managing is how much of the old glass is left in the pot when the new one goes in. That’s dilution, and you already know how to handle it.
COE is overblown too
Compatibility gets pinned on a single number — the coefficient of expansion, or COE — and the community treats matching COEs as the whole answer. It isn’t. The founders of Bullseye Glass made this point decades ago: expansion is only half of it. Viscosity matters just as much, and glasses with an identical COE can still fail to fit. Two products both stamped “96” guarantees nothing on its own.
Here’s the part nobody says out loud: you already run on trust. Suppliers print “96,” and you take that number on faith — you don’t send your own glass out to a lab to confirm it before you melt. And the number shifts a little in your shop anyway, with furnace age, a wearing pot or liner, and normal variation from one delivery to the next. The colors you pull scatter around the range; they aren’t all an exact 96. You melt it, you make work, it holds. Glasma asks for the same trust you already give every day — nothing more.
So coming off ordinary soft glass, COE is not the thing standing between you and a switch. Treat Glasma the way you already treat your current glass: trust it for daily work, and test it when the stakes are high — a critical color pairing, fusing, or tight coldwork, where sometimes a batch just doesn’t fit and the only way to know is to pull a test and look. That’s not a switching problem. That’s glass. The one real exception is genuinely dissimilar glass — borosilicate, lead, an off-COE art glass — where the numbers do need checking first.
What about the parts I already made?
The second thing studios ask: “I’ve got cane, cups, murrini — a whole shelf of parts made from my old glass. Will they still work with the new?” Fair question, and the honest answer is short.
We won’t pretend to know. We don’t have data on your specific parts against your specific glass, so anyone who promises they’ll fit is guessing. It’s the same truth as above: the COE in the catalog doesn’t settle it, and the only way to actually know is to test — combine a little of the old part with the new glass, anneal it, and look for stress or cracking before you commit a real piece.
The good news is you don’t have to find out the hard way, mid-piece. You can check first, two easy ways:
Test on a shop already melting Glasma
There’s a decent chance one is near you. Bring a few of your parts, pull a small test combination on their glass, and see for yourself — no need to fire up your own pot of Glasma to find out.
Send your parts to us
Mail us a few pieces — some cane, a cup, a bit of murrini — and we’ll run a compatibility test against Glasma and tell you what we see. Depending on what you need, that’s either a small fee or on us — just ask.
Either way you get a real answer instead of a guess — before you risk a finished piece.
It’s like rinsing a coffee cup
Pour out the coffee, fill with water, and it’s mostly clear — but there’s still a hint of coffee in there. Rinse once more and it’s gone. Your pot works the same way. You empty out as much old glass as you can, fill with new, and the first melt runs mostly new with a little of the old still mixed in. Run it down, top up again, and the old glass fades to nothing.
We’ll be straight with you: we can’t tell you exactly how much carries over. It depends on your furnace, your glass, and how empty you got it. A round pot you can nearly drain; a rectangular day tank holds glass in the corners you’ll never reach. So you plan for a transition melt or two rather than expecting a clean cutover — and you’re never caught out.
How the switch goes
No teardown and no new pot required. The whole job is: get the old glass as low as you reasonably can, fill with new, and use the transition melts on work that forgives a little cord.
Empty out as much old glass as you can
Work the level down and gather out what you can’t use up. In a round pot you’ll get most of it. In a rectangular day tank you can’t reach the corners, so you won’t get it all — that’s fine. Getting every last bit isn’t the goal; getting it low is.
Fill with new batch
Charge Glasma into whatever’s left and bring it up to a melt. Because the pellets are fully and evenly mixed, they melt down clean and often faster than loose batch — so the new glass comes up to working order quickly.
Run the first melts on forgiving work
There’s going to be some old-and-new mixing in that first pot, which usually shows up as cords. So don’t schedule your best thick clear blanks for that day. Blow production pieces, color, textured or smaller work — anything where a little cord won’t show — and save the showpieces for once it’s clean.
Rinse and repeat if needed
Work that melt down and top up with new again. Each round the old glass is a smaller fraction of the pot. One pass clears most setups; a heavily-loaded tank, or one you couldn’t empty well, might want a second. After that you’re fully on the new glass and you never ran an empty furnace.
Where to actually be careful
Color to clear. This is the one switch that’s more than dilution. Strong color contaminates clear at tiny fractions, so you want the old glass as gone as possible — plan extra rinses, or time that switch for a reline when the furnace is empty. Going the other way, or clear to clear, is the easy case.
Thick clear work. Cords love a thick clear blank. Keep your finest clear pieces off the transition melts until the glass reads clean.
Dissimilar glass. The reassurance above is for soft-to-soft. If you’re coming off boro, lead, or an off-COE art glass, treat compatibility as a real question and check the numbers.
Reading & fighting cords
Transition melts throw cords — it’s expected and temporary. If you want to read them, chase them, and know when the glass is clean again, the cords guide is the companion to this one.
The melting method
Once you’re on the new glass, a clean melt is all that’s left. Here’s the charge-watch-sample rhythm that gets the most out of a pelletized batch.
The official specs
For the authoritative COE, cullet-mixing guidance and melting recommendations, go to the source — kept current by Glasma.
Thinking about switching?
Tell us what you’re melting now, what your furnace looks like, and what you make. We’ll walk you through exactly how the dilution would go for your setup — how many rinses, what to run in between — same as we do on the phone.
Talk through your switch