Testing and Measuring -
Let's start with the quick and easy test and then go to the more
complicated, difficult or more time consuming ones.
The one I always use is called the...
- Two equal amounts of glass are melted together.
- Pull the melted glasses into a thread.
- Let it cool
- Measure the bend (if any) in the thread.
- If the thread is straight when cooled, the glasses match.
- If the thread is bent, the glasses don't match.
- The more the thread bends, the bigger the mismatch.
- The higher expansion glass is on the inside of the bend.
- There are some glasses this doesn't work very well on.
- It's not extremely accurate.
1. Two equal amounts of glass
are melted together.
easiest way I've found to do this is cutting about 1/2" wide strips of
clear sheet glass from a known and tested expansion (COE) sheet of
Spectrum 96 for 96, Bullseye for 90, Moretti for 104. Spectrum and
Bullseye test their glasses, I assume Moretti doesn't test their
glass because it
can vary a little, but not too bad. This way we have one glass that we
are sure what it is.
Note: I think Frantz Art Glass is about the only place to
get Moretti sheet anymore.
Melt some of the unknown glass to be tested on to the strip of known
COE glass. You're trying to get an amount on there that will be the
same size and depth as the part of the strip it's covering.
it flat with the tweezers so you can get a good idea how much
glass is on the strip of known glass. It's much easier to judge how
much when dealing with the same shapes on each part.
Be sure you've got the same amount and thickness.
2. Pull the melted glasses into
Melt these two together . They need to be very liquid so that we have
enough time to pull them into a thread.
Grab the melted glass with the tweezers and pull.
I always turn the strip of
glass at a right (90 degree) angle to the thread so I can remember
which section of the
melted glass was the strip and which was the glass I added to the
Keep pulling on the thread until it cools so you're not putting any
extra bend into by letting it droop while hot.
We need a 200 mm or 7.87" long section of this thread to measure the
expansion so the thread has to be pulled a bit longer than that.
sorry I don't have a pic of the actual pull. I couldn't hold the camera
and do the pull.
3. Let it to cool.
Break the blob off the end where the tweezers were so its weight
doesn't throw us off when we go to measure the thread.
4. Measure the bend (if any) in
I've cut this piece of aluminum angle iron to exactly 200mm, or 7.87
This is what I use to measure the bend from from.
I use an inexpensive set of measuring calipers. They're accurate enough.
I measure the distance between the bent thread and the angle iron by
eye, being sure I'm looking straight down. You know you're right when
you see only the top of the angle iron, not any of the side, like in
the pic below.
5.If the thread is straight when
cooled, the glasses match.
6.If the thread is bent, the
glass do not match.
7.The more the thread bends, the
bigger the mismatch.
more than a 1.5 mm bend in the thread means that glass can cause
mismatch trouble in a finished glass piece.
There are some interesting
exceptions and workarounds to this. We'll go over some of these in a
minute, and some
others later, at the end.
You can't read the calipers in the pic but this one is at 1.5 mm,
so it's just OK.
8. The higher expansion
glass is on the inside of the bend.
The higher expansion glass is going to expand further than the lower
expansion glass. It's also going to shrink more when it cools. This is
going to force the lower expansion glass to bend along with the
higher expansion glass as it shrinks.
This is what's making stress inside a piece of glass when the glasses
don't match. Deep inside a piece of glass there is nowhere for the
lower expansion glass to go to compensate for the mismatch. These
threads are thin enough to bend to compensate. This also gives visual
proof that glass is elastic.
9. There are some glasses this
doesn't work very well on.
The two glasses tested need to have about the same amount of
elasticity. Technically that's called/measured as the modulus of
I know, "elastic" is not a word you think of with glass,
particularly when it's cold, but it's there and it's important.
Lead makes glass a LOT more elastic. it's been used for many
years (centuries?) in large quantities for colored glasses so they
could get away with larger mismatches and still have the glasses hold
together. It works unless you try to saw the glass, then it will break.
If you try the thread test with one lead glass and one regular soda
lime glass your results will be off. Way off.
This is also true for all the striking glasses, fluorine whites,
phosphate whites, reds and yellows and silver glasses.
You can get these troublesome glasses to work with a thread test, you
just have experiment and find out how far the different glass throws
the result off and factor that in. That's done by multiple trials and
checking the results for stress with a polariscope.
With many of the striking glasses you can figure out the number of
strikes needed to make the piece you have in mind and strike the
combined glasses melted together for the thread test that many times
before you pull the thread. This helps increase accuracy a lot.
10. It's not that accurate.
Not accurate like laboratory tests, no, but it's fast, costs next to
nothing and it's more than accurate enough to check what you're doing.
More important, you can find and fix trouble before it happens or use
it to figure out what has to be done to fix a known problem. We'll get
into that when we go into E&T factors and using them to fix
Some other common tests for glass mismatch are:
1. For furnace glass, blowing out a cylinder with an equal thickness of
both glasses, one inside the other. You anneal the cylinder, score the
cylinder, then crack it open the score. The two ends will either stay
put, open wider, or try to close on themselves depending on how much
mismatch there is. No movement of the split means the glasses match.
Not many people are skilled enough to get the two layers of glass equal
thickness. It's tough to measure how much you're off. It takes a long
2. Fusing the glasses together in a kiln and then looking at them under
a polariscope. A final check with a polariscope is always a good idea
for any of these methods.
Running the glasses through a cycle in the kiln takes a long time.
There's an article about homemade polariscopes here