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mirror blank molds

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#1 danjones

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Posted 18 August 2013 - 09:24 PM

hi. i've been casting my own mirror blanks. two important things i learned early on, i need a mold that can take the heat, and contain the molten glass. i'vee been using plate glass mostly making thick blanks on account of the thermal expansion, which another discussion. i'm in testing with pyrex, with promising results. i'v used many materials for molds. i have found that the molds i've used can only be used once or twice. i've used kiln brick, steel, ceramic fused materials, with the best results in stainless steel. the top dog for mirror molds is refractory cement. the first pic is my own recipe for the cement, portland cement mix, sand, crushed kiln brick, and not in the shot about a quart of premixed refractory cement. the other pic is the resulting mold. this one was just an 8 inch for testing, but was great. i use bigger molds of this now with great success. i can get many castings per mold before it begins to break down from the high heat. lend me your thoughts on reuseable mold. thanks for looking.

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#2 danjones

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Posted 18 August 2013 - 09:26 PM

here is the resulting mold

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#3 Pinbout

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Posted 18 August 2013 - 09:31 PM

Is there any draft angles. Could you use a two part mold?

#4 Achernar

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Posted 18 August 2013 - 10:09 PM

Couldn't you use cast iron? If I am not mistaken, that material has been used for molten glass.

Taras

#5 danjones

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Posted 19 August 2013 - 07:14 PM

i think i've seem some of your vids on youtube pinbout. i did use some 2 part molds. i used kiln brick sitting on a kiln shelf, steel on kiln shelf, and ss on kiln shelf. even wired down to the shelf with ss wire, the glass somehow squeezes its way under that seam, like lava, building up more and more until it flows out. i've also tried these molds on a steel shelf. even the ss begins to corrode badly after only a few firings. they are under that high heat for as much as 50 hours during annealing and dont seem to handle it. i've test fired a peice of iron, and it didnt handle it that badly, not great. more importantly, trying to shape the iron to a mold is much work. i dont really have any iron around here already shaped for mold use. that may be a possiblity if one can shape it easier. i had a ss pot that worked well, but began erroding badly after only 4 firings. much better results than the other molds tho. two part molds, the glas seems to find its way in that seam. not always leaking, but sticking the mold together. i'm using at this point the refractory mold lined with a thick ceramic felt, which has had much success. times per use per mold...to be determined. great potential so far tho. with the cement mold, of course it has to be mixed, poured, cured, pre fired for durability test. bit of a process, but with good results.

#6 Pinbout

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Posted 20 August 2013 - 05:27 PM

When I was in college way back when, we had all kinds of furnaces, kilns,. I blew glass and poured molten bronze. I wish I had those tools now. For the molten brass cadtings we made mixtures of hydrocal and sand. We never casted glass just watch some vids.

#7 danjones

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Posted 20 August 2013 - 08:28 PM

'melting' glass is the easy part, other than finding a mold that is reuseable many times. this one is a trial, but has now survived 2 firings unchanges. has potential. the hard part is annealing. i anneal for nearly 50 hours which includes the cooldown to ambient. my home built controller does all the work, programmable, wifi enabled also so that i may control it from inside at my pc away from my shop. wireless webcam to keep a check on it thu the nights. sweet set up.

#8 Pinbout

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Posted 20 August 2013 - 09:30 PM

i anneal for nearly 50 hours which includes the cooldown to ambient.



I never got into the glass blowing as much as I should've to play with the kiln to anneal them. My teacher did that.
[Phila. College of Art]

#9 TxStars

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Posted 21 August 2013 - 02:28 AM

You might try Boron Nitride which is used in molten steel processing.

#10 steveastrouk

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Posted 21 August 2013 - 06:39 AM

Zypcote. Com make a great variety of furnace grade paints. One of the problems with fully molten glasses are that they are very good solvents of metals! Several of the customers for my day job... designing instruments to test molten materials, use pure platinum. Sadly I don't get to keep the scrap.

#11 danjones

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Posted 26 August 2013 - 10:16 PM

here is the result of the above mix. failed. actually, i would have been surprised if i got it right on the first try. no problem, tho, this is just one out of a few recipes. one will work.

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#12 danjones

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Posted 26 August 2013 - 10:18 PM

this mold took the heat til about 1100F. not quite good enough. i need it to withstand temps around 1800-2000F.

#13 Pinbout

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Posted 26 August 2013 - 11:41 PM

What I made for bronze is called investments. Plaster and sand one shot deals . That brings me to sand casting. Although they're one shot it wouldn't be hard to prepare it for another use?

#14 Pinbout

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Posted 27 August 2013 - 09:01 PM

I was just reading on atmfree group about how much hydracow.

When I did any plaster work I never measurd weights , even with the investments.

I would supersaturate the water with our schtuff. Sifting as i added to the water until I saw mountains of dry material all over.

So my point is maybe the ratio of water is wrong?

#15 steveastrouk

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Posted 28 August 2013 - 05:27 PM

Have you tried castable refractories ?

#16 danjones

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Posted 30 August 2013 - 04:43 PM

this was actually a home brew for a castable. i had a blonde moment the other day. ill keep working on a home brew refractory, but ill drop that for now and simply use normal firebricks, like for wood stoves. i actually use them in my oven now to set the mold on top of for leveling, durability. fired them many times and no change to them. ill just zip out the shape with my diamond saw blades, refractory them together with a premix i have, and done. many times reuseable. will post a pic when done. gonna start on it tonight.

#17 danjones

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Posted 30 August 2013 - 09:48 PM

so heres a pic of some test pieces of the firebrick from menards. the one on the left was my first attempt. it stands 4.5" tall that way. thought if i could cut the curve out, it would take less bricks, and have a nice tall mold wall. didnt work out so well, hard to do with an angle grinder. the pic on the right is the one i'll go with. i cut them in half lengthwise, then cut out the curve on both, then stacked them up. i'll cut many like this, glue them together with premix refractory, clean up that inside edge to smoothe it out, and have a solid 2.5' high mold wall. the floor of the mold will be these bricks too. need to make a trip to menards for more bricks.

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#18 kfrederick

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Posted 31 August 2013 - 06:35 AM

What size mold you making ? Hope it works out .Mark and I own 110 lbs of pyrex trimmed off out blanks .How big can you do? What you have planned for the two thick 8s?

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#19 gatorengineer

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Posted 31 August 2013 - 06:39 AM

Dan there is a yahoo group called Red Hot Mirrors, not super active but contains a lot of information on casting mirrors. Hope this helps.

#20 steveastrouk

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Posted 31 August 2013 - 12:37 PM

Great link Mark, thanks for that...I just joined...

#21 danjones

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Posted 31 August 2013 - 10:07 PM

hi kevin. i can do up to 16" in this kiln. ill build a new bigger kiln in the coming spring, and do up to 20" in it. no bigger beacause my aluminzer can only coat up to 20". as for the 2 8 inchers you sent, i'm leaning towards making a 1.75 inch, big as i can get. havent done the math yet to see if they can make a 20. thanks for the link gator, i'll check that out.

#22 danjones

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Posted 31 August 2013 - 10:11 PM

here's the firebrick mold i'm working on. i have cut out several arcs out of the brick, stacked them up in two layers, may do a third high, and stuck them together with premixed refractory cement. need to set overnight, then i'll just arrange them for the circle, and cement them down on a firebrick slab.

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#23 danjones

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Posted 31 August 2013 - 10:32 PM

interesting pyrex kevin. i had a post a while back about pyrex bakeware being borosilicate glass. of course, its a big controversy today, but i did manage to find out some facts, and with my own testing. i read several articles that stated pyrex was actually pyrex before world kitchen bought them out. it was stated that a flame test on the old pyrex would tell the truth in that. crush up some of the old pyrex clear baking dishes, torch them, and if the flame had a green tint to it, it was borsilicate. sure enough, i found some. the boron in the glass tends to emit a greenish glow.

#24 danjones

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Posted 31 August 2013 - 10:42 PM

gator, send me a link to that group if you would. cant seem to find it exactly. thanks.

#25 steveastrouk

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Posted 01 September 2013 - 11:54 AM

interesting pyrex kevin. i had a post a while back about pyrex bakeware being borosilicate glass. of course, its a big controversy today, but i did manage to find out some facts, and with my own testing. i read several articles that stated pyrex was actually pyrex before world kitchen bought them out.


European Pyrex IS still boro-silicate though - I actually approached the makers (who are now French, but with a big distribution in the UK) and they were happy to sell me tonne lots of shop damaged stock - we didn't get to the point of negotiating how much, because the project didn't get to lift off speed.

Steve






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