This is a full-thickness, 8 inch diameter telescope mirror blank that I made myself. This web page will show how I did it.
Here is the Duncan kiln I got at a yard sale for only $95. A great deal! The kiln only has a 12 inch inside diameter, which will limit the size
of the blanks I can make, but it can be powered by any 120 Volt outlet, which is very nice. I have no 240 Volt outlets at home or in my workshop.
I prowl thrift stores looking for cheap glass. I'll often buy glass tabletops or shelves to cut up. Here is a huge pile of glass
tabletops I bought for only $15. It is a mix of 3/8 and 5/16 glass in round and square pieces. The smallest piece is the round one at
24 inches in diameter. What a bargain. It must be over 100 pounds of glass. It will keep me happily experimenting for months.
Thrift shops often get glass tables or display cases that are damaged or incomplete, but the thick glass is in reasonably good shape.
The thrift stores will usually sell damaged or incomplete pieces quite cheaply. They are usually just happy to be rid of it. I always
ask if they have any thick glass in the back room that they are willing to sell. The thicker the better. If it has any chips or nicks in it, that's
fine, because then I know it isn't tempered glass, and it can be cut.
Here is a stack of four 8 in diameter disks I cut out of 5/16 inch thick glass. I've gotten quite good at cutting circles out of glass.
It's taken a lot of practice though.
My early attempts at fusing glass taught me that I was going to need a mold to constrain the glass and prevent it from running. I don't really
want to heat the glass up until it flows like water, but just enough to soften it so the layers fully fuse together. However, at the high
end of the fusing temperature, the lower layers of glass will get squeezed out like toothpaste by the weight of those above. So a mold is
needed to constrain the glass and keep it in a disk shape.
I made an 8 inch diameter hole cutting saw out of a piece of 8 inch steel AC duct and some scrap steel strapping. I welded an old drill-mount
nut driver to the center of it so I can chuck it up in a drill. It only took a few minutes to build, and it works very well for cutting the
I used a metal nibbler to cut teeth into the edge of the hole saw. This makes the saw cut through the soft firebricks like a hot knife
through butter. I was amazed at how well this improvised saw cut the bricks.
Here I have two bricks held in place in a wooden frame, ready to cut with the saw. The saw was a little tough to start since I didn't
include a pilot drill bit. Since the bit would be in the seam between the two bricks, it wouldn't be much help anyway.
So I cut some small pieces of plastic and screwed them into the corners of the bricks to center and guide the saw. The screws easily bite
into the soft bricks. Once the saw gets a good bite down into the bricks, I take off the plastic guides so I can drill down to the
full depth of the saw. It is amazing how easily the saw cuts the soft bricks.
This is the finished mold. As well as drilling out the center, I cut off the corners so I could get the mold in and out of the kiln without
scraping the edges and damaging the kiln walls.
Here is one of the kiln shelves. It will be the base for putting all the pieces together.
I start by putting a piece of kiln shelf paper on the shelf. I'm using Bullseye Kiln Paper. It is infused with ceramic. The paper burns away
in the kiln and leaves a thin layer of ceramic powder behind that the glass will not stick to. Molten glass will stick to anything it touches, and
will need to be chiseled off once it cools. So kiln wash or kiln paper is absolutely necessary to prevent the glass from sticking to the shelf.
I prefer the paper. The porous kiln paper also lets any air trapped under the bottom of the glass disks escape and not rise into the soft glass
as a big honking bubble. Place the paper on the shelf with the printed side down.
Here I have stacked four 5/16 in thick glass disks in the center of the kiln shelf. The glass disks have been edged to true them up a little
bit, and cleaned by running them through my dishwasher. Once cleaned, I only handle them by the edges. The idea is to keep them clean so that
there is no oil on them, or specks of dust or dirt that would hold the disks apart and keep a film of air between the disks.
Here I have wrapped thin strips of kiln paper, printed side out, around the stack of disks. This will prevent the molten glass from
sticking to the mold. The strips are held in place with tiny pieces of Scotch tape. I am always afraid that residue from the burning
tape will cause problems, so I use as little as possible. Just enough to hold the strips of kiln paper in place until the mold is
clamped around the glass.
Here the two halves of the mold have been put in place around the stack of glass disks.
I put a couple of wraps of stainless steel wire around the mold and twist them tight. This is to prevent the hydrostatic pressure of
the molten glass from pushing the mold apart. I've seen it happen. It is amazing how much pressure an inch and a half of semi-liquid
glass can exert on the mold. Wiring the mold shut is essential.
The whole assembly is then placed in the kiln. It is hard to tell from the photo, but it is sitting on three kiln posts, and not on the
floor of the kiln. The kiln has been carefully leveled. This is important. If the kiln is not level, the semi-liquid glass will flow
until it finds its own level, and your finished blank will have a lot of wedge to it. My kiln needs to have one leg wedged up about 3/4
inch to make the kiln floor level.
Now it is time to fire the kiln. I start off with a very slow ramp up of around 400 degrees per hour until the temperature exceeds 850
degrees. This is to prevent thermal shock from fracturing the glass disks. Then I crank up the temperature as quickly as the kiln will
heat up to about 1250 degrees. I let the glass soak at this temperature for about 45 minutes. This soak minimizes the amount of air
trapped between the layers of glass. Once the soak time is up, I once again crank up the temperature, as fast as the kiln will heat up,
to 1700 degrees. You want to get the glass through the 1350 region as quickly as possible. This is the temperature where devitirification can
occur. So I have the kiln at it's highest setting to get to 1700 as quickly as possible. As you can see by this photo, I over-shot the target
temperature by a little bit. Let the glass soak at 1700 for at least 10 minutes. This will ensure a complete bond between the layers.
Here is a peek through the vent hole of the kiln with the temperature at about 1700 degrees. I have experimented with various temperatures.
1700 may seem a little high for glass fusing, but I find it makes any air trapped between layers form into little round bubbles, rather than
remain as sheets between the layers. I have experimented with trying to increase the temperature to 1900 or even higher in an attempt to reduce
the viscosity of the glass to the point the bubbles will rise to the top. Unfortunately, the glass begins to run out of the mold before it gets
runny enough for the little bubbles to rise very far. So I no longer go much above 1700. Using thick glass ensures that the first layer of bubbles
will be deep enough inside the blank that it won't be reached when grinding the mirror.
This is what it looks like inside the kiln at about 1700 degrees. I am trying to cool the kiln down rapidly after the 10 minute soak at 1700 degrees.
I get the temperature down to about 1200 as quickly as possible. This is again to minimize the time spent in the devitrification region of 1350 degrees,
and to simply reduce the length of time the run takes. I open the kiln with a long pole and wearing elbow length thermal gloves. A face shield would
also be handy, since the heat coming out of the kiln is very intense. My kiln is outside on my patio with nothing nearby that could ignite or be
charred by the heat (except me). I turn the kiln off and open the kiln for 15-20 seconds (or as long as I can
stand the heat blast) several times to allow it to rapidly cool down. Once it has cooled down to about 1200 degrees, I close the kiln and let it
continue to cool down naturally until it reaches a temperature where the glass is cool enough to handle. This usually takes about 24 hrs.
If everything worked all right, I get a rough blank that just comes right out of the mold and doesn't stick to the kiln shelf. I've had things
go horribly wrong though, with the hardened glass stuck to mold and shelf. Since I perfected the above procedure though, there haven't been
any more bad incidents. This is one of my early blanks made of many thin layers. The blank is quite rough around the edges. It takes on the
shape of the mold, which is rough and full of voids. A little glass always squeezes out around the bottom of the mold and leaves a ridge
(seen at the top here). There are also always two vertical parting lines where the two halves of the mold come together. One can be seen
to the right of center here. It doesn't look like much in the photo, but it is sticking out a couple of mm from the edge of the blank. I've
thought about ways to make the inside of the mold smoother, like coating it with a thin layer of refractory cement to fill in the voids. I
may try that in the future. For now though, I use a grinder to smooth out and true up the edges of the blanks. I figure that even with a
smoother mold, the blanks will still need a little grinding work, especially at the bottom ridge and parting line.
Here is a blank with the rough edge ground down and trued up. It only takes a few minutes on the grinding machine I built for the job.
Here is the super simple grinding machine I built to true up the edges of the blanks. It is just an angle grinder with a masonry wheel
mounted on the edge of a table. I also mounted a lazy-susan bearing on a piece of plastic between fixed guides. This allows me to move
the bearing toward and away from the grinder. Only a few minutes of spinning the blank against the spinning grinder wheel gets rid of the
rough edges and parting lines, and makes the blanks truly round.
The glass blanks have a lot of internal stress that needs to be relieved. This is accomplished through annealing the blanks. Annealing involves
heating the blanks back up to 900 degrees slowly so they don't break due to thermal shock. The mold is not necessary, since we aren't getting up
to temperatures where the blank might soften. The insulating properties of the mold might also inhibit proper annealing. Soak the blank at 900
degrees for about four hours. Then allow the blank to cool very slowly, about 50 degrees per hour, down to 800. Then I turn off the kiln and let
it cool to room temperature naturally. This annealing cycle relieves all the internal stresses in even thick blanks.
This particular blank was edged, then returned to the kiln for fire polishing and then annealing. To fire polish the blank, it is heated up to about
1300 degrees in the kiln. At this temperature the glass won't melt, but it will flow enough to smooth over rough edges and sharp corners. Once the
fire polish was done, I lowered the kiln temperature to the annealing temperature and did a regular annealing cycle. I don't
bother fire polishing all my blanks. I just wanted to try it with this one to see how it looked. The fire polish made the rough edge nice and smooth
and transparent. It is possible to look through the side of the blank and see the individual layers of glass and the planes of bubbles. It's neat.
Now that I have several blanks made, it is time to grind and polish one into a telescope mirror and test it out. I can use one blank as the mirror and another as the grinding tool, and have a couple in reserve in case something goes horribly wrong during the grinding and polishing operations. I'll post an update once a mirror is completed.
Ok, so this process isn't really quicker or easier than my friend's trepanning saw, However, my kiln is smaller and quieter than his saw, and I don't have to buy or scrounge for hard to find, full thickness glass to cut into blanks. I can use cheap and or free thinner glass.
I am on the lookout for a larger kiln that will allow me to make blanks in the 10-12 inch size range. If anyone knows where I can get one cheap, please email me and let me know.
Here is the first 10 inch blank out of the new mold. Much better than the one above. The blank came out very round and didn't stick
to the mold causing the sort of bad chips I saw with my first mold. That first mold worked so badly that I'm not even going to show any photos
or write up how I made it. I don't want to send people down a dead-end street. The mold I am using now is the way to go. It works so much better.
Here is the result of firing the above stack of glass. Once out of the kiln, I removed it from the mold, and trued up the edge on my grinder.
It is a fine-looking, near full-thickness, 10 inch diameter blank that I intend to grind and polish into a 1st class telescope mirror.
Here is a stack of my home-made 8 and 10 inch mirror blanks. The second from the bottom is the one above.
I can use the thinner blank below it as the grinding tool.
I started by making a 14.5 inch inside diameter mold. I used a hot wire foam cutter to cut out a 14.5 inch diameter circle in
styrofoam. Then I cut circular segments out of soft K-23 firebricks with a bandsaw to encircle the foam disk. Later I used furnace cement to glue
all the pieces together. I then coated the inside of the mold with kiln wash to prevent the molten glass from sticking to it.
I cut three 14.5 inch diameter circles out of a piece of 1/2 inch thick glass tabletop I bought cheap at a thrift store. I have found that the
glass needs to be very clean to get a good fusion between the layers. So I always clean the glass in my dishwasher before putting it in the mold.
These disks were so tall that I had to remove the upper rack from the dishwasher to get them in.
I disassembled the kiln to make loading the heavy glass and delicate mold into it easier. No more leaning down into a deep kiln to assemble the mold,
and scraping my knuckles on the sides as I work. I love this new kiln. I took off the two heater rings, leaving just the base. I first
carefully leveled the base. I started with a shelf that was well coated with kiln wash, sitting on three feet. I added a piece of shelf
paper, just to be safe. Then I stacked up the three pieces of glass and clamped the mold around them.
Here is a view down into the kiln after reassembling it around the mold. The object sticking in from about 7:00 is the thermocouple
temperature probe. I positioned it right above the mold in an attempt to get an accurate gauge of the glass temperature. The
kiln was now ready for firing. The entire firing cycle took about 7 hours. I used a very slow ramp up to prevent stressing out and
breaking the large mass of glass. There was also a long soak at 1200 degrees to eliminate air between the layers, and a soak at
1700 to ensure a good fusion. Due to the very large mass of glass, cool-down took a very long time (over a day) before I could handle
the blank, and remove it from the kiln.
So here is the completed blank after removing it from the mold and truing up the edge on my grinder. The blank is 1.5 inches thick and
weighs 20 lbs, 10 oz. This isn't the biggest blank I can make in the new kiln. I should be just able to make 16 inch blanks in it, but it
will be tight. I'll need to find some more thick glass.