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View Full Version : a simple How To make Hagy Seals


Mike McCain
12-17-2017, 11:09 PM
Someone explain the right way to do a Hagy seal test for me, and who Hagy is/was.

Pete VanderLaan
12-18-2017, 08:14 AM
Make a rod of your clear glass, of whatever host glass you use. Make it 4mm in diameter. Draw out a cane of your other glass you want to test the same diameter. Cut the canes to about 1.5 inches in length. Place two of the canes of the clear on the outsides of the color cane like you were making a sandwich. Now with a torch, fuse the ends of the cane to each other- just the ends. Now anneal it and bring the cane out of the annealer and put it, completely cooled on the table of a good quality polarimeter with the lower plate in full retardation axis with back light. Holding another piece above the subject cane, observe the seal and begin to rotate the film or plate you are holding in your hand. The cane color will begin to darken and at it's darkest point you can measure the number of degrees of rotation it took to gain that amount of retardation. If you get more than five degrees, you have a mismatch. Depending on where you rotate clockwise or counterclockwise determines which way you are off.

The real difficulty here is measuring those degrees. My Polarimeter came from Strainoptics and costs over two grand. Croucher has the same unit. It measures by single degree in either direction. Not a tool you can really justify owning unless you make color for a living. Not unlike owning a crusher. The other thing about hagy Seals is that if you are not quite close on the mismatch, the seal simply breaks and it does it easily. Then you have nothing and it's why I continue to prefer the ring test which is indeed more subjective than the Hagy seal. The seal is absolutely on the money.

In the ring test, take a gather of your color and case it in your clear. Blow out a thin cup and anneal it. When cool, place it in a diamond saw and cut off a ring of the cup. If it actually survives this act, chances are good the two glasses are compatible with each other. A disturbing number of color rods will fail and break at this point if worked with a so called 96 LEC glass. But if it does survive, make a vertical score across the ring with a glass cutter and tap open your score. If the ring opens with a small gap, it's telling you that the interior glass has a lower expansion than the outer glass. If it tries to overlap the opposite is true. The trouble with this test is that you now know you have an issue but since you don't make your own clear, or color, there's nothing you can do about it. At least you know what kind of long term bombs you may be producing.
In general, Gaffer really has the best track record. Kugler opaques have the worst with most opaques testing around an 89. Reichenbach is good on transparents. Effetre can be incompatible with itself. It's the clear manufacturers that are the real source of trouble. When they change, the earth shakes from breakage.

Hagy was a materials Engineer at Corning. You can read more about him here:
https://www.cmog.org/library/glass-seals-and-thermal-expansion-compatibility

You will find the seal discussed very thoroughly in that ever popular yet hard to find book "Glassnotes IV by Dr Henry Halem. It is out of print. I suspect Henry could be coerced to supply the pdf pages on the seal for a fee. You could ask him. Frank Wooley and I really covered the gamut of testing for mismatch in that book. I don't think any serious glassworker should be without it. It's only in the 4th edition. Owning the 3rd is worth it as it had a good deal of good stuff that was removed in the 4th.