These are the cores being fired to sinter the alumina rigidiser, took quite a while to get the center of the cores up to 800'F!
Ok, after a hiatus caused by discovering my love of glass art I'm now back on
track with my "cellular mirror development program", this casting attempt will
push both my limits and the limits of the kiln, but if it works the pay offs
will be well worth the risks. After all I can always build another kiln!
What I'm hoping to gain from this casting run is:
A. How does a ceramic fiber mold hold up to the rigors of glass casting?
B. How well does a large blank anneal in a small space?
C. Is a fiber mold re-useable?
Now I do know that this is the material used at the Big Mirror Lab at the University of Arizona, but I don't know what the fixative is they use (and they wouldn't tell me when I asked!), so I used a colloidal alumina rigidiser purchased from Wesbond Corporation. which is rated to a higher working temp than the hot face ceramic fiber I used to build the mold from. I coated all internal surfaces with a glass releasing kiln wash, just to make sure that the Pyrex won't react with the mold.
So I hear you ask: What does this mold look like? Well here are some pictures....
These are the cores being fired to sinter the alumina rigidiser, took quite a
while to get the center of the cores up to 800'F!
A trial assembly, nice to see that everything fits, more or less as designed, a
little tweaking was needed but on the whole quite a painless process, so far.
The mold is ready for the kiln!
Here is the
mold in the kiln and the required Pyrex, 7.5Kgs (some 15lbs) to be exact,
loaded.
Ok, so I had to open the kiln to help it keep track with the cool down cycle
prior to entering the anneal program and I took the following photo then. Some
preliminary results: The mold material shrank about 11% (not measured, just a
visual observation), but otherwise the mold appears to have held up quite well
with no other apparent problems. The upside to the larger than expected
shrinkage (I was expecting 1.5% as per the product specifications) is that the
mirror will be slightly larger, but the face plate might be a little thinner
than expected (I was shooting for 13mm remember). One nice thing, there doesn't
appear to be any bubbles present, at least in the area imaged anyway!
BTW the temperature when this picture was taken was 1040'C (1850'F) in the kiln,
not a nice thing to have your hand and arm near, even with a Kevlar glove!!!!!
Doesn't look like much does it, but it has taken me some 6 years of research and experimentation to get this far, and I won't really know how well I'm doing until I open the kiln again in about two weeks time. Wish me luck, and check back in two weeks time as I'll post the results of this experiment then, good or bad.
Well the two weeks have come and gone, and when I opened the kiln this is what I saw:
As you can see there is a lot of scale from the stainless steel mold ring I
used, so that is not a great idea I'll have to think of another option, maybe I
can get my hands some Inconel 600 sheet stock to make a replacement ring out of.
Anyway now to answer the three questions I asked at the beginning....
A. The fiber held up incredibly well, although it did shrink more than I was
lead to believe it would, in all about 8% shrinkage was measured, so I should
have added (had I known this!) about 8% extra glass, the upside is the blank is
8% larger in diameter!!!! The other thing I noticed that the metal ring deformed
the mold by about 6mm or 1/4", so the blank is slightly out of round.
B. I can't answer this as I have decided to re-melt the blank, with an additional 200grams of glass, there are two reasons for this, one is I removed the metal ring, so the mold should (in theory) relax back into the true round shape it start out at, the risk is that the mold might let go and leak hot glass into the kiln. The other reason is that as you can see in the picture there is some divitrification present, you can see how hazy the cores appear, right? You might also note that one of the edge cores has shifted slightly (nothing to be done about that though). Well when the blank hits the melting temp this time, I'm taking it up a further 111'C or 200'F to a peak temperature of 1121'C or 2050'F, yes that's really hot! I hope that some of the devitrified material will re-dissolve back into the bulk of the glass, if not then being lighter than the parent glass it will float freely to the surface and can be ground out when the glass gets that far. The while streaks are small vanes of alumina dust from the kiln wash which I now know I don't need, they too should float up to the surface.
C. I now realize that this type of mold is not re-useable......
Well what can I say? I got some of the answers I wanted, and some I really didn't want nor expected! What will I find in 13 days time? Well I expect (read that as hope) to find a near perfectly round blank that has little devitrified material remaining and is ready to be turned into a great mirror.
So please check back in about two weeks time to see if I'm right this time! I will post a picture of the blank at start of the anneal cycle, like I did last time, so this picture will be up in about 24 hours or so..... Or not, sorry about that!
To see this picture and all the post casting results etc just click on the
next tag below.
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