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Porcelain Mirrors

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Posted by Stephen Campbell on November 2, 2002 11:41:20 UTC

Anyone know anything about the use of porcelain as a substrate material for a telescope mirror?,
ive been doing some research on the subject, ive found out that a 24" mirror was made for an observatory in china and it worked well, porcelain consists of kaolin clay,and feldspar and flint as fluxing agents,but sometimes has addional materials such as ball clay,cristobolite,
bentonite,etc.
the coefficient of thermal expansion varies depending on the percentage of the various materials anywhere from 1x10-6 to 15x10-6 but usually around 6 or 7 x10-6.
Porcelain is a vitrified, semi translucent ceramic, i believe it is possible to make a mirror with this material so long as it obeys the following criteria:
1)must be vitrified, as non-porous as practicable.
2)has a coefficient of thermal expansion preferrably less than 4.00x10-6.(pyrex is 3.25x10-6).
3)is glassy,and therefore polishable.
4)microroughness:this is my primary concern, being composed primarily of crystaline complex silica(clay) there may be too much of a rough surface to be suitable as a mirror substrate.

most of you no doubt will dismiss this idea right away, but think about it, its basically simular to low-e borosilicate glass, a silicate and a fluxing agent and high temp fusing is involved.
from what i understand the kaolin(clay) is partially dissolved,the kaolin is the crystaline component, the feldspar is the flux which partially dissolves the clay and produces the "glassy" component, there are all sorts of problems with this process(of coarse), if an "astronomically correct" formulation could be developed this would indeed be an ideal material for use as a telescope mirror,no doubt about it.
I am looking for this formulation and i'll keep you all posted as to what i find.


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