Basic Stretch Test Coefficient

[CelticGlass]

I was asked today to try and help out with glass mismatch issues that can drive you nuts and no matter what you seem to do, small cracks still appear that make you despair. I hope this very simple test will help towards checking some basic compatibility.

This is not a panacea for all coe issues as to what fits with what, under a heap of differing conditions. However, it does clearly show some strain conditions and where you least expect to find them.

The first picture shows you how to make side by side rod end beads of similar size and then using a pair of large tweezers or pointed pliers, pull down a two rod stringer to a length around 6 to 8 inches. do this vertically and avoid lateral sag from gravity. Hold for about 10 to 15 seconds.

Now lay them on your bench and note the curvature.

"Note" FH Members only can see the embedded step by step images

[CelticGlass]

You'll note  as in your school chemistry days, just as the bi-metalic strips in a spring would pull in a particular direction, so does the glass.

The softer high coe is on the outside longer radius.... Oddly though this ASK104 Blue appears softer than the pink coe 104 Lauscha

The black and white and the green and red are all coe 104 Lauscha and remain quite straight and are not pulling against each other.

The very last picture shows how extremes of coe difference can occur, I've run a pyrex rod and coe 104 clear into a stretch test and the stringer arced severly and very quickly, until it exploded after around 15 seconds of cooling.

[CelticGlass]

So if your having flowers or small surface embellishments hopping off the bead, it could be even quite small differences, as in the ask104 and the pink lauscha coe 104.

A 3 degree difference sometimes brought about by the metal oxides or the batch manufacturer, can be sufficient to cause issues. Lets be fair, the manufacturers often give a 3 points of coe tolerance. If this is the case and your using two different makes of glass and each has a 3 point coe shift in different directions, you can be putting a 6 point stress coe into your work.

If all else fails or you have doubts about what your trying to fit together..do a stretch test.

Good luck