RE: [ATM] Making glass blanks.

["Timm Simpkins" <poduck@poducksworld.com>]

While it is possible to find automated temperature controllers on ebay, the
fact that I have no experience with temperature controllers makes me
hesitant to purchase one.  $65.00 is a good price, and I don't know that I
could make a redneck engineered one for less, but one issue is teaching
other people how to do it, as well as getting it to work.  I want what I am
doing to be repeatable for everyone.  Since new controllers that I know
would work have constantly been in the $200-$250 range, I have hesitated to
purchase one.  If someone only wanted to make a 10 or 12 inch mirror, that
kind of price would make it more expensive to make their own blank than to
buy one. (This is assuming that a person makes only one blank.)

An electric self cleaning oven reaches temperatures between 900 and 1100
degrees Fahrenheit normally, and this isn't the best bet.  1100 degrees
barely reaches the upper annealing point, and unless it's known what
temperature you can consistently get with a particular oven, it might be a
gamble to start converting some ovens.  Also, an oven has a lot of space to
heat.  The larger the space, the more expensive it becomes to heat it.  On
the other hand, if you were to take the oven apart, you could use the
elements to heat a smaller space to much higher temperatures for the same
cost.

As far as buying kilns goes, I've been making my foundries for years, and I
have made a couple kilns.  The kilns weren't more than a temperature control
and a timer, much like an oven has.  While making these, I have found that
there is a simple and cheap process for making a sort of castable
refractory.  I use fire clay, volcano cinders, washed silica sand, sawdust
and water.  This makes a sort of concrete mixture.  I use the volcano
cinders to add extra air space, to offset the cost a bit, and to add
rigidity.  Since cinders are free for me at the local neighborhood dead
volcano it cuts my total cost in half.  The sawdust burns away after the
first firing and leaves additional air spaces.  All this air space adds
extra insulation.  This mixture is very inexpensive, and a 2-3 inch
thickness is all that is necessary to insulate.  The fact is, I can make 5
kilns the size and shape that I need them for the price of 1 premade kiln
that won't do close to what I want to do with it.  The case is made from
sheet metal, and that's where a lot of people will have to outsource some
work if they don't have their own sheet metal brake, but it shouldn't be too
expensive.  Anyway, elements and all I've spent less than $100 to make the
entire kiln, and if I really scrounged, I could have made it for less than
$50.  For someone who wants to do smaller mirrors, like 12" or less, you
could make two kilns from a 50 gallon drum.

I'm interested in doing everything I can for as cheap as I can, and as
repeatable as possible.  After all, the mirror is the most expensive thing
that most ATMs invest in when building a scope.  If we can all make our own
glass, it would put amateur astronomy in the hands of more people.

Also, I'm talking about manufacturing my own borosilicate glass, not
slumping or even fusing it.  This process takes a bit more heat than just
remelting premanufactured glass.  Currently, people are taking glass plates
and fusing them together and/or slumping them.  While this is one way to do
things, premade glass is much more expensive than sand and borax.  Since the
annealing process is the same for fused glass and for totally homemade
glass, the cost difference would be in the price of materials and the
relatively small difference in power costs for reaching the higher
temperature for a short period.  Also, quality could be more closely
controlled.

To give some ideas, premade glass must reach 1300 - 1400 degrees just for
slumping. (This is where I doubt a self cleaning oven would work.)  In order
to fuse glass, it must reach 1500 degrees.  After this, the glass needs to
come up to about 1700 degrees so that the air bubbles can come to the
surface.  The process I'm using needs to exceed 2000 degrees for a short
time.  As soon as the silica melts, the temperature can be brought down to
around 1700 degrees for a couple hours until the bubbles have all come to
the surface and popped.  After this, the temperature is brought quickly down
to the upper annealing point by removing the lid of the kiln, the
temperature is then held there.  When the entire thickness of the glass has
reached this temperature, the kiln needs to be very slowly brought down to
the strain point, where glass is solid and its structure is unlikely to
change unless heated again.  Since I'm planning on helping other people do
this, and I have no idea what the annealing point of their glass will be, it
is necessary to use a "shotgun" annealing process.  That way the temperature
is brought down slowly over a wide range of temperatures so that all
annealing points are covered.

I want to detail the entire process and make it available for everyone.
While purchasing a temperature controller would work, it runs the risk of
defeating the spirit of the project in the first place.

Someone suggested a variac.  This too would be a bit on the pricy side for
what I'm trying to do.  I have known about variacs for a while, and have
read that after WWII they were snatched up from army surplus for use in
annealers.  This is basically what gave me the idea to think about a dimmer
switch.  A variac would handle 7.5 kilowatts, but at the prices these go for
new, they would be a problem.

Nobody should misunderstand me.  If these methods are the best way to go, I
will probably end up using them, but I'm looking for a method that is as
close to duct tape and bailing wire as possible, while still maintaining a
proper annealing process.

Thank you,

Timm Simpkins


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