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If you're not interested in all the technical details of "why",  or you just want to know how to check if one glass will fit another, skip this first part and head over to Page 2  for  "Testing and Measuring"

Ever notice that most commercial glassware is only one color? That's because it's hard to put two different colors of glass together hot without having problems. Art glass is usually the only place you'll  see color combinations done hot.

The problem is stress where the different glasses join.
This stress will cause cracks, breakage or even explosions in the glass  when it cools, or, sometimes much later, when it's sitting in someone's house.

There are a number of things that create stress.

First on that list is...

Coefficient of expansion or COE

COE means coefficient of expansion.

Big words but a simple idea.

It's a measurement of how much a glass expands (heating up) or contracts (cooling down)through a certain range of temperature. COE is measured in ten thousandths of an inch .

In art glass, almost everyone uses the English & Turner system for finding, calculating and correcting expansion. Much more later about E&T.

The temperature range used to measure the expansion of the glass in the English & Turner  system is from 20 to 300 degrees Centigrade.

A 96 COE glass is one that expands or contracts 96 ten thousandths of an inch as it heats up or cools down through the range between 20 and 300 degrees Centigrade.

These are the most common COEs seen in art glass today, and the manufacturers best known for them:

90 COE = 90 ten thousanths of an inch expansion - Bullseye Glass

96COE = 96 ten thousandths of an inch expansion - Spectrum, Gaffer, Spruce Pine

104 COE = 104 ten thousandths of an inch expansion - Moretti

If you're batching and melting your own glasses you can change the COE of your glass.

In case you didn't know, "batch" is what glass melters call the mix of sand, alkalis,lime, coloring agents and other odds and ends that gets melted down into glass.

We'll start with....

What is Glass?

Basic glass is sand, alkalai to lower the sand's melting temperature, and calcium (lime) to help harden the glass and keep all the alkalai from dissolving out of the glass when it gets wet.
Soda ash is the most common alkalai used so that's where the term "soda lime" glass came from.

Add more sand and the COE goes down

Add more alkali or calcium and the COE goes up.

There are a lot more chemicals that can end up in glass to change its characteristics. If you're going to add these others you need to know if they raise the COE or lower it.

That's where the E&T system begins to shine. It tells you how much COE change a certain amount of a certain chemical will cause.

Many think that the only time two glasses will create a mismatch, and the resulting stress, is when the COEs aren't the same. Unfortunately, that's wrong.

Why the Big Emphasis on COE?

Due to a lot of misinformation left over from an earlier era of US art glass and over-simplification by some glass marketers over the years.

That's why so many people think COE is the only thing that has to match for different glasses to fit together without stress.

Sometimes you have to make the COE of one glass very different from the other to get them to fit together without problems.

All the glasses like System 96 or Bullseye 90 are not all 96 or 90 COE glasses. They are a system of glasses that will fit each other within that system without problems. There will be different COE glasses within that closed system. 


It's a long story, get comfortable, and we'll start with...

Annealing and Strain Point

Annealing point
Is the temperature at which the glass is just about to turn from liquid to solid, or the other way around if you're heating it up instead of cooling it. It's just liquid enough to flow at this point.
Whatever you are annealing isn't going to collapse or distort because it's close enough to a solid to hold its form.

Strain point
Is where the glass becomes a solid.
Hot glass above the strain point is liquid and plastic. It will flow and has no stresses.
Once glass cools to the strain point it does not flow any more and it's done expanding, contracting or flowing.

When cooled off and solid, after passing through the strain point, glass is both elastic and brittle.
Elastic meaning it can bend but will return to it's original shape, like a spring.
Brittle meaning it breaks cleanly with no distortion at the break, like ice.

Let's start with the problems you can get into with annealing

Any glass worked hot must be cooled slowly enough to anneal correctly

Means cooling the glass slowly enough to be sure the temperature all through the glass stays almost equal while cooling. This gets tricky because glass is very slow to take on or give up heat. The thicker the glass, the slower the heat penetrates or leaves the glass.

Annealing Stress

Even without mismatched glasses combined in the same piece, you can put big stress into a piece of glass by not annealing correctly.

Let's say we're annealing a piece of glass and it's all the same glass.

  1. If the temperature drops too fast, then the outside of the piece cools to the strain point and sets up before the inside does.
  2. The inside and the outside of the glass will be trying to pull away from each other because the outside quit contracting while the inside was still shrinking.

Another way to think about that...

  1. The outside is being held at a larger than normal size by the expansion of all the glass, inside and outside.
  2. The outside becomes solid at this larger-than-it-would-normally-be-when-cool size.
  3. Now the inside of the glass is trying to shrink down to its normal smaller size as it cools but it can't.
  4. Huge stress builds up as the inside tries to tear away from the outside

Mismatched glasses stress

  1. One of the two glasses goes from liquid to solid (gets to strain temp.) before the other as they are cooling, Once you get at or below the strain temperature of one of the glasses, there's no flow and no more shrinkage in that glass.

  2. The other glass joined to the first glass hasn't hit it's strain point temperature yet so it's still shrinking.

  3. Stress builds up as the second glass tries to pull away from the first. when the second glass hits its strain point temperature (becomes solid) then the stress from it trying to pull away from the first glass is locked in.

Even though it's all about the strain point in both cases, annealing problems and mismatch problems are often mistaken for each other. If you're having problems that you think are mismatched glasses then one of the first things you need to check is your annealing cycle and times. Run a piece of clear of the same type and size through the anneal process and then check it for stress with a polariscope when it's done. If this comes out OK then you can be sure you've got a mismatch and not an annealing problem.

Expansion and contraction happen in very tiny increments, which is why we measure them in ten thousanths of an inch.

The stress they create when they mismatch or are not annealed correctly are NOT tiny, even at just a few ten thousandths of an inch.

These stresses make huge pressure, as in tens of thousands of pounds. Which is why artwork or any glass with mismatched glasses and/or poor annealing can explode like a bomb.

All those plate glass table tops and other pieces that have been exploding at random lately are caused by melting the glass with inexpensive ingredients that contain something that will cause tiny "stones" in the glass.

The stones are a combination of nickel and sulfur that melt together and form little stone-like pieces of incompatible glass inside the finished glass. These stones have a VERY different strain point from the rest of the glass. Even though those stones are tiny, they make enough stress to shatter a piece of 1/2" or larger plate glass.

Strain Point is where the stress gets set into the glass permanently if things aren't right. Right means any different glasses have been correctly matched and the annealing was done correctly.

If you are combining different glasses, you might think the perfect situation would be for both glasses to have the same strain point, and you're right.

We'll get into exactly how to get that done and what it is we're really changing as we look into...

The Fix

You're usually trying to adjust a colored glass to fit the clear glass you use.

If there is a mismatch, what you have to do is change the COE of the colored glass to get it to fit the clear.

You end up with two glasses that are different COEs but that fit each other with no stress. This happens because the strain point temperature of both glasses is the same and that's what really makes a go or no go out for combining glasses.


Bear with me...It's story time again, starting with the history of trying to get glasses to fit each other.

Before some very generous glass scientists started helping hot glass artists by teaching them the finer points and "why" of annealing and matching glasses, a lot of glass matching was done by a "rule of thumb". That rule said that as long as you didn't add more than 1% of anything that made glass color to one of the glasses, that the glasses two glasses would fit together well enough.

Superstition has always been a problem with glass info. Coming into the twentieth century many factories were still melting glasses by the phase of the moon.....which has no effect.

You still hear this "1% rule of thumb" today so let's look at where it's right and where it's soooooo wrong...

The rule is partially right because the transparent colors aren't usually a mismatch problem. They take well under 1% of the oxide used to color them to get the desired result. This 1% or under is usually put into the same formula batch recipe that was used to melt the clear so the strain points and everything else stay close enough that they fit together without problems.

The 1% rule is wrong because some glass color additives will cause plenty of trouble at well under 1%.

Fluorine, Phosphorous, Silver, selenium and/or cadmium (red and yellows) and many other chemicals used to make striking glasses will cause problems at low percentages, way below 1%.

Striking glasses are the ones that change color, color density, or opacity when they are cooled and reheated. Mostly opaque glasses, silver glasses, reds and yellows.

The 1% rule is also right if you're melting very dense transparent colors, like the color bars furnace workers use. The percent of added colorant used has to go well over 1% and then there's trouble...big trouble.

So how did  hot glass artisits have as much success as they  did in matching glasses in the past?

Well, they didn't actually solve the problem, there was a "patch" to get around the problem...

Many of the dense colors for furnace glass workers in the past, and even now, are full of lead...as in they're 40% lead.

Lead does incredible things to increase the elasticity of glass. As a nice side benefit, it makes it pretty too.

There can be a large mismatch in strain points and the lead will allow the glass to stretch enough to relieve a lot of the stress....unless you try to saw it, that is. There's enough stress left in the glass to cause it to break if sawed.

The problem was "patched" but wasn't really solved this way and lead is one nasty toxin.

So why all the trouble, workarounds and strange problems?

Because not many people then or now understood the effects of changes in......


Viscosity is how easily a liquid will flow, or, the more correct definition, reistance to flow.

If the two glasses you're trying to match have the same COE but have different viscosities, there will be a different strain point for each of the glasses.

There will be permanent stress left in the glass when it's cooled down and done because they didn't hit the strain point together.

Annealing has no effect on this, it can't change the strain point so it can't relieve this type of stress.

A lot of the problems with the troublesome color additives mentioned above are because they make BIG changes in the viscosity of the glass when tiny amounts are added. 

The same problem happens with over 1% regular color additives, the viscosity starts to go crazy at that point.

These viscosity changes are not linear or predictable. As in...doubling the amount of color additive doesn't necessarily double the amount of viscosity change.

 It gets even crazier with the striking glasses because the molecular structure AND the viscosity of the glass change radically at the point where they cool enough to strike. This is usually right close to or at the strain point. Each seperate strike after the first (some glasses need more than one strike to bring out the color) changes the structure and the viscosity again. 

Complex problem, right?

Yeah, it is, so let's get into some more detail on how to fix it, assuming you're melting your own glasses and can change the chemical makeup to change the COE.

Fortunately, changing either COE or viscosity change will change the strain point.
Changing the COE changes the strain point in a gradual, linear and predictable way.

Remember all the crazyness in viscosity we talked about before with a lot of the additives that make color?  No tracking that, right?

You need something that's predictable to test and correct with.

To put it another way...

When you get it right you can have two glasses with different COEs and different viscosities but a match on strain points...and they will fit each other without stress.

Let's go over to Testing & Measuring  for some how-to.

Want to read more, and I mean a LOT more about this?

Read a LONG discussion among many of the old hands in the art glass world about how the COE "mistake" happened here