[ATM] Re: 40" mirror -- Glass terms

["Mike Conran" <mike@guidesys.com>]

This may be helpful for those of us unfamiliar with the specifics of certain 
terms in glass work.

From: 

http://users.ticnet.com/mikefirth/glosothr.htm#S 


"Anneal, Annealing Temperature 

When glass has been heated to a high enough temperature, it must be cooled 
by a controlled schedule of time and temperature or it will crack simply 
because of the strain inside the glass from the outside cooling faster than 
the inside. The sound of cracking glass in the waste buckets is common in 
glass studios. 

The thicker the glass, the longer the cooling time must be, days and weeks 
in the case of really thick castings. The annealing temperature is 
determined by slowly heating a long thin piece of glass supported at the 
ends until it just starts to sag (the sag temperature) and the annealing 
temperature is taken to be 50°C (90°F) below that, usually about 900F 
(480C). 

When scientific measurements are possible, the annealing point is a specific 
viscosity.  "annealing point, AP—the temperature corresponding to a rate of 
elongation of 0.0136 cm/min when measured by ASTM Method C 336, Test for 
Annealing Point and Strain Point of Glass by Fiber Elongation. This test 
prescribes a rate of cooling of approximately 4 C/min with a fiber of 
approximately 0.065 cm in diameter, and a suspended load of 1000 g. The 
annealing point numerically approximates log = 13.0 poises, where internal 
stress is substantially relieved in a few minutes. 

www.kimble-kontes.com/pdfs/glass_fabricating_terms.pdf" 


"Slumping 

Mostly the same as sagging, with the possible proviso that sagging is 
sometimes limited to work done into a mold while slumping is more narrowly 
the heating of unsupported glass, so glass may be slumped through a ring 
(drop out mold) or over a stainless steel form. 

Softening Point
the temperature at which glass has a viscosity of 10 7.6 poises. In this 
temperature range glass will deform noticeably under its own weight: ASTM C 
338. "softening point, SP—the temperature at which a uniform fiber, 0.55 to 
0.75 mm in diameter and 235 mm in length, elongates under its own weight at 
a rate of 1 mm/min when the upper 100 mm of its length is heated in the 
manner prescribed in ASTM Method C338. Test for Softening Point of Glass at 
a rate of approximately 5 C/min. For glass of density near 2.5,  this 
temperature corresponds to a viscosity of 10 7.6 poises. " 

www.kimble-kontes.com/pdfs/glass_fabricating_terms.pdf 

 

 

 


Jerry B. Hillman writes: 

> Hi Matt,
> I am looking into Cas, coude, etc. designs, but haven't learned enough about
> them to make a decision to go that route. This is only my third mirror.
> Succeeding on the simplest terms, ie. a Dob/Newt is probably going to be a
> challenge as it is without trying the exotic designs. 
> 
> Clear skies,
> Jerry 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
 


Mike Conron 
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