Glass blowing molds

Dear Sirs Hi I am looking for some info on glass molds for blowing. I work for a large Lamp Co in Miami Fl one of our dept is a glass blowing and slumping.Myself I only know a little about glass as my background is mold making ceramic, silicone and the like. Even thou I helped set up the glass dept and build the glass tanks and use to charge the glass at night before leaving that is about as far as my knowledge goes. When we first started to do glass we were melting marbles in side our ceramic kilns. I was making the one use waste molds for the glass out of plaster EPK and vermiculite some one found the mix in a glass book. That guy is long gone and I don't have the formula any more it has been like 9 years since we needed molds like that. The glass dept moved on to machined Graphite molds and metal molds. So my hands have been out of the glass for around 6 years. Do you have any idea were I can find the formula? (plaster EPK and vermiculite mix) Book web site or do you know of it and can please you share it.

Next they are looking at the cost of the metal molds we have to have made 2 to 3 thousands dollars each and they are looking to find a cheep way to make the molds so I was ask to look for fast easy ways to make the molds. I have some ideas and I am wondering if they will work or if you or any one you know has tried them. All the ideas relate to glass blowing molds. All molds will be from 1 to 4 parts.

1.Ceramic molds like used in slumping the mold will be in 3 to 4 parts. Because of undercuts. 2.Refractory like ZerCar can it be molded and will it last for say 50 blows. 3.Plaster EPK vermiculite mold. 4.Polyurethane mixed with metal powder 5.Epoxy mixed with metal powder 6.Graphite molds is there a way for me to make mold using it. I Would like able to pour it. 7.Do you have any ideas that may be of help.

Thank You Rodney Rock Master Mold Maker / Design Fine Art Lamps Miami Lakes Florida Phone 305-821-1055 EX 3065 Work Email Home Email Web Address

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The thing about mixes is there isn't a magic thing where it will work, as opposed to one ounce off and it will suddenly shatter exactly three hours into setting.

I've never heard of the mix, but gee, just grab some plaster, clay and vermiculite and start mixing.

I don't know what the requirements are, but personally I'd drop the clay: it's only going to cause shrinkage on trying.


-- "California is the breakfast state: fruits, nuts and flakes." Website:

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Reply to
Tim Williams

When I worked at the former RCA Laboratories as a co-op student, I hung around a lot in their glass shop. They mostly produced vacuum tube envolopes, but these have a great deal of similarity to light bulb envelopes.

They used machined graphite molds for their blown envelopes, and graphic templates to shape those turned on a glass lathe.

The tube headers and stems were purchased from an outside vendor, so I don't know how these were made.

Harry C.

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In which case, you should probably tone back part of your .sig for a while -- the part which reads:

====================================================================== Master Mold Maker ======================================================================


Actually -- what little I know about glass blowing was done without molds, and was in chem lab class -- just a bit of starting glasswork.

Do you *know* that this is used? It strikes me that the temperatures would be way too high for something like this to be any more than a single-shot mold.

Similar feelings about these -- except that they might handle two or three -- depending on how well cooled the mold is.

*Pour* it? Graphite is a crystalline form of carbon. According to my old copy of the _Handbook of Chemistry and Physics_, carbon's melting point is 1350 C, and it *sublimes* (that is goes straight from solid to vapor form) above 1300 C, so if you were to melt it to pour (neglecting whether it would form the right crystal form on cooling, instead of diamond or some amorphous form), you would need to do so in an inert atmosphere, at some significant pressure. The specific gravity (and thus relative density) is:

amorphous 1.88 graphite 2.25 diamond 3.51

so my guess would be that it would be some variant of amorphous without a very carefully controlled pattern of temperature and pressure.

Just to put it into perspective, that melting point, which was given in degrees C (1350) translates to 2462 F -- well above what I can reach (1850 F) in my metal hardening oven.

In contrast, glass has a temperature of fusion (close enough to a melting point for our purposes) of 1100 C (2012 F). And, of course, different glasses will have differing melting points -- but this is probably close enough to make my point, and it was what I could easily look up.

Obviously, if glass melted above the melting point of carbon (graphite) it would be of little use to you as a mold.

For the shaping of the graphite molds, you will want to start with sold blocks of graphite, and machine away parts to make your mold. You may be able to do it with a lathe, but a milling machine would be a better choice -- and a CNC milling machine would probably be the best choice to get precise shapes. This will require someone to learn how to run and to program one.

You *could* use a burr in a die grinder to cut the shapes by hand, but this would make it more difficult to make the halves (or more parts) of the mold meet properly. This might be reasonable for an experiment, but not for production.

And since carbon dust from the machining can be rather abrasive, depending on how pure the graphite is, you may want to get a CNC milling machine which is nearly worn out anyway (which will reduce the acquisition cost, if not that of maintenance).

And they could be used to machine the metal molds as well (and they would probably live longer doing that.)

I don't know what metals are a proper choice for glass molds, but it would have to be something which could run at a temperature near that of the molten glass, so you don't get a sudden cooling and fracture your workpiece.

Good Luck DoN.

Reply to
DoN. Nichols

If they are indeed used, I would suspect the binder burns out (*cough*) on the surface, leaving a somewhat temp-resistant layer of filler.

Whoa! You missed a rather large factor there, DoN. ;-)

Next closest thing I see on my periodic table is either terbium, dysprosium or silicon. The latter makes the most sense since it's right below carbon.

As for carbon itself, it melts/sublimes (nearly the same at 1atm pressure) somewhere around 4000°C, or about 7150°F. Just a *little* bit hard to cast...

Would be neat if molten carbon crystallized into diamonds, but unfortunately that takes a lot of pressure, AFAIK. Although I think they've been getting around the extreme pressure or temperature things with CVD (chemical vapor deposition), I should read up on the current tech (if it isn't a total trade secret).

Within reach of a good foundry furnace, though. My graphite-based crucibles aren't falling apart in the heat, so I think you grabbed the wrong number ;-)

Actually, I'm not sure if purity is the cause. IIRC, "graphite" is made artificially by heaping ground charcoal or coke, lampblack, other junk and pitch together into an electric furnace. The pitch decomposes to either amorphous or graphitic carbon, binding together the mass; the other material is recrystallized by the intense temperature, forming a polycrystalline block. I've heard the structure described as nodules, globs, flakes and other junk. It's still graphite, but it's kinda random I guess, and that's why it's abrasive.

I imagine natural graphite (probably damned expensive) would be wonderful stock to machine.

Eh- pressed glass is made in warm (dark red heat, probably 1200°F) cast iron molds. Worked glass is formed with steel and moistened wood tools! (Lichtenfrost effect, anyone?)


-- "California is the breakfast state: fruits, nuts and flakes." Website:

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Reply to
Tim Williams

*Pour* it? Graphite is a crystalline form of carbon. According to my old copy of the _Handbook of Chemistry and Physics_, carbon's melting point is 1350 C, and it *sublimes* (that is goes straight from solid to vapor form) above 1300 C, so if you were to melt it to pour

Don I don't want to melt it to pour.I am thinking in a cold cast way Graphite powder mixed with polyurthane,epoxie, or some other type of binder. So I would beable to cast the mix have it set up and be able to blow glass in it. Snip: In which case, you should probably tone back part of your .sig for a while -- the part which reads: Master Mold Maker :-) It took me a lot years to get that title.As well it only relates to ceramic molds for ceramic castings,Silicone for resin castings and latex for plaster castings.But you may have a very good point. thanks Rodney ;)

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Hmm ... I really think that something like that would have serious problems as anything other than perhaps a one-shot mold -- and even then I would expect the epoxy or poly-whatever to outgas massively on contact with the hot glass surface -- both cooling it too quickly (risking shattering), and perhaps deforming the workpiece with bubbles formed from contact with the mold's surface.

Good Luck, DoN.

Reply to
DoN. Nichols

You're right. One of the problems of having to put on glasses to read the small type in the _Handbook_, and having to take them off to see the screen at about 24" distance from my eyes.

Re-reading I find 3550 C and 3500 C (6422 F and 6332 F respectively).

I mentioned the problem of it subliming below the melting point. And, it looks as though you used one of the extended ASCII characters for degrees, which displays as '\260' on my screen -- one reason for avoiding using *any* extended ASCII character -- not everybody sees them the same.

And we still have a disagreement of melting points (though not as extreme as before). Mine is from the _Handbook of Chemistry and Physics, 43rd edition_, (1961-1962). "(Description of) The Elements", starting page 405, with Carbon on page 410-411. The first few lines read:

"Carbon (L. carbo, charcoal), C; at. Wt. 12.001; at. no. 6; m.p. 3550 deg C, sublimes above 3500 deg C; b.p. 4200 deg C; sp. gr. amorphous 1.88, graphite 2.25, diamond 3.51; valence 3, 3, o4 4."

I don't think that the characteristics of Carbon, or our knowledge of them, have changed that much since 1961. :-)

Forgive me for the use of HTML tags in there, but I was not sure of any way to show italics to everyone -- some newsreaders may actually

*display* that as italics -- the rest will cue the reader that they should be italics.

So -- what is the source for *your* figures, now that I have corrected mine?

Yep -- sometimes achieved by surrounding the carbon with steel which shrinks in the chilling, generating the forces necessary -- but only for very small diamonds.

That would be interesting. I wonder what happens with vapor deposition -- assuming that the "boats" can get hot enough to evaporate carbon. If you started depositing it on a seed diamond, and on something with a matching crystal lattice, the crystal might grow nicely.

I did -- or at least typed the wrong one.


I suspect so.

You mean floating the glass above the wood on a film of steam?

Enjoy, DoN.

Reply to
DoN. Nichols

My Optometrist set me up with a set of glasses recommended for data entry people.They are basically upside down bi-focals with the small mid range lenses in the upper portion. It took me a couple weeks to get used to them, but they work great for me and I even use them when reading in bed. Gerry :-)} London, Canada

Reply to
Gerald Miller

Well -- I've got bifocals, too -- but I work with the screen far enough away so I don't need the glasses for that. The close work inset is on the bottom, for detail work, and the main parts are tuned for the distance to my lathe chuck while standing. They *are* true safety glasses, for this very reason.

But you know -- back when that book was new (1961), the type was a *lot* larger than it is now. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

The binder would probably have to be clay or refractory. I just had an idea. What about ceramic shell molds modified to do what you want? They probably would have to be heated. Karl

Reply to
Karl Vorwerk

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is an Australian foundry that has developed a specialised high temperature steel for glass molds.

Drop them an email and see if they can help in your application.

Hope this helps, Peter

Reply to
Bushy Pete

Yeah, but you're using that weird Unix thing that nobody else ever uses.

AFAIK, most any windows or Linux reader displays ALT+0248 at the degree symbol, so I'm reaching most of my audience properly, and you, the minority, must cope. :-p

Yeah, as I was typing I thought 7,000 seemed a bit hot... I always heard it in the middle 6k range.

- Since your post is in plain text format, OE doesn't even try to translate any HTML, and it gets as ugly as my use of an extended character.

Flinn Scientific periodic table, has various data alongside each element. I might've screwed something up, since it lists temperatures in kelvin, but I think I subtraced 273 properly, so *shrug*... FWIW, it says 4188K melting point, which should be 3915C = 7079F - go figure. It quotes, for instance, Al M.P. correctly at 933K = 660C = 1220F, dunno if others might be screwed up.

I thought the iron was only necessary as a catalyst, by dissolving it at a lower temperature? High pressure cast iron, anyone?

Well, come to think of it, the tools have to apply some force, so I'd guess the heat of vaporization is used instead. That way, only a few thou's of the surface gets charred, rather than a quarter inch deep and cracked. Less smoke too, though more steam and cooling.


-- "California is the breakfast state: fruits, nuts and flakes." Website:

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Reply to
Tim Williams

Now remember -- we've been through experiments before, and differing systems show different characters, in part depending on what characterset is selected. And while most recent systems have a choice of charactersets (and I'm using iso8599-15), but there are still people who don't have a choice.

I'll bet that more people than you think will be scratching their heads at whatever they see on their screens.

Of course -- I was depending on *people* understanding what the "" pair meant -- especially since I also explained it immediately following.

Is that specific for graphite, or general for carbon? Mine was general, with no separate figures given for graphite, so that might be the difference.

I think that the shrinkage of the iron helps in the formation -- perhaps just that slight extra boost over what they could get from their hydraulics.

Enjoy, DoN.

Reply to
DoN. Nichols

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