Why Plaster Molds Crack

[David Paterson]

I want to comment on the issue of why plaster/silica molds crack.

It seems to me that a mold can only crack from internal stresses. In plaster/silica molds these stresses can only come from heating, and could generally come only from these causes:

(1) Escaping moisture from a damp mold, or from the release of chemically combined water. In this regard, plaster only releases part of its chemically combined water at low temperatures. A little research came up with the information that plaster doesnt release all of its combined water until it reaches over 1100 degrees F. I am not sure if most people are aware of this.

(2)Since the molds are generally 50% or more silica, the quartz inversion range is obviously another possible cause of cracking. If you think about it, with half the mold made of silica, which will all expand at once, it is perhaps a miracle that at least some molds dont crack.

I experimented with molds while at school, and tried substituting wollastonite (calcium silicate) for silica, because I didnt want the exposure to silica. The mold worked fine and I got a good casting.

It seems to me that there must be other cheap materials that could take the place of silica, without the inversion problem, and possibly even giving a better surface finish to the glass.

In this regard, I know that some people put koalin or other ingredients into the mix, and koalin also has chemically combined water, as do some forms of alumina.

Some people recomend adding talc to the mix for a face coat. Has anyone experimented with making a mold that is plaster and talc, or plaster and some other mineral?

[Lauri Levanto]

As I mentioned earlier,
I have used plaster+Aluminium Oxide for the
inner coating of molds.

-lauri

[ellen abbott]

David

I've made molds of plaster and talc. I've also tried plaster and anything else I could get my hands on. I just use plaster and silica now. I don't use face coats either. I try to keep it as simple as possible.

Quartz inversion, in my opinion is a non-factor. If you dry the mold right, on the way up, anything will work.

Marc (Ellen's tech guy)

[charlie holden]

Ted Sawyer recently talked to a class I was in about water burning out of molds. He put a thermocouple into the center of the plaster of a mold and recorded the temperatures of both that thermocouple and the kiln interior. He soaked the kiln at about 225 F (I don't have my notes in front of me.) The temp inside the plaster stayed below 212 for several hours. When it rose to 225, meaning that all the loose water in the mold had steamed out, he ramped up the kiln.

The temperature of the plaster stalled again at around 350 F while the kiln continued to ramp up. He looked into it and says that the people at US Gypsum say that is where most of the chemically bound water is burning out. I'm not sure now whether there is more water burning out at 1100 or not. I've seen it written several places though.

I think quartz inversion can be a problem if the plaster and silica are not mixed well. If you have a clump of mostly silica it is going to expand more than the material around it. Rember also that the silica isn't expanding all at once, since different parts of the mold reach that temperature at different times.

I also think that a mold can crack just because it is not strong enough to hold back the weight of the glass. That can be a function of any number of problems -- too much water, too thin walls, poor mixing, uneven support from below, etc.

[ellen abbott]

I hate to get crosswise with people teaching. But I think we tend to get bogged down on the wrong details.
If you can't mix your ingredients well, then you need to do a better job mixing. If your mold cracked under the weight of the glass, then learn to make stronger molds. If your mold cracks in the drying stage, dry better.
In our pate de verre molds, I use twice the silica than plaster.
I run it up to 300°F and hold for 3 hrs. Then I go up to 1100°F at 100°/hr and hold for another 3 hrs. Some of my molds are 2 lbs and some 70 lbs. Same material, made the same way, fired the same way will yield the same results.
Every material is not going to be suitable for every application. There is no "one size fits all" when it comes to molds. Good casting practices will take you further than searching for the "Moldy Grail".

Marc (Ellen's tech guy)

[rodney]

--------------------------------------------------------------------------------
MARC WRITES:

In our pate de verre molds, I use twice the silica than plaster.
I run it up to 300°F and hold for 3 hrs. Then I go up to 1100°F at 100°/hr and hold for another 3 hrs. Some of my molds are 2 lbs and some 70 lbs. Same material, made the same way, fired the same way will yield the same results.

Marc (Ellen's tech guy)


HEY THERE MARC,,,thanks for this info,,,,could you go ahead and finish off your firing schedule,,,all the way out to the end of the annealing, if you dont mind,,,thanks for all the help,,,,,,,,,,,,,i think you have a good team, you and ellen,,,,over here its me, myself, i, and the wgbb

thanks
rodney

[ellen abbott]

Rodney
I don't mind telling you exactly what I do. (Apparently I'm going to make you ask twice) But you missed my point. What I do probably won't work for you. What I do in my paragon kiln won't work in my home-made kiln. Every small change between what I tell you and what you use and do will will make this unworkable. If I came to your place, I would expect to have to try this in your kiln a couple of times before I got it right. It might take many firings to get what I want.
Ellen and I use good casting practices. I can't stress that enough. We know how to make cast sculpture. We apply this knowledge to our glasswork. We also are not afraid of failure. We try things without expectations.
So, I guess, my point is art is not science. You've got to dive in an do it.

[John Kurman]

We also are not afraid of failure. We try things without expectations.
So, I guess, my point is art is not science. You've got to dive in an do it.

Not to T-Bone this thread, but, hold on there. Empiricism is exactly what science is all about - at least, since Galileo, which makes it a very, very recent mode of thinking.

Remember, before people tried things without expectations (or tried things despite expectations), we in the West all followed what Aristotle had theorized about the world. And he was, for the most part, an enormous butthead who was completely clueless about how the world operated. Then, fortunately someone decided to test the dogma, and wham, some progress for a change.

I'm not sure what you are describing when you use the word science, but it ain't science.

I know quite a few scientists, both experimentalists and theoreticians. And you wouldn't believe how often I've heard this statement when I've pressed them for the deep-down dirt on what they are doing: "You know, what I do is still pretty much a black art".

End of rant.

[ellen abbott]

I have experimented and I have theorized, but I'm not a scientist. The best analogy that I can come up with is literature. Writers don't try different combinations of letters trying to find new words. They don't try different symbols to find new letters. They combine what is common.
All our molds are made from common materials easily obtained. This is our vocabulary. Learn this first. Then learn grammar.

[John Kurman]

Marc,

I didn't mean to send this thread off track, or ruffle your feathers... if I did. You compare what you are doing to literature. Fair enough, as far as analogies go.

My point, though, is that what you are doing is science - or at least what you are doing is in the best of scientific traditions. You figure out a method, test it, see if it works, abandon or refine it, and learn from it. I'm not sure when the divide between science and the humanities occurred, but it is a false distinction. Artists are Scientists. No doubt about it. They explore, they play. No scientist, as far as I know, hardly ever (if ever!) uses the old Scientific Method we all learned in elementary school. You know, form a hypothesis, blah, blah, blah.

Nope. Scientists typically start from a framework of what they know, and then either ask "I wonder what happens if I do this?" or, by accident, see something unusual occur when they botch a procedure. All the big discoveries have been serendipitous. No one was ever looking for them beforehand. No expectations.

Science, for the most part, involves play, play of the freest sort. The scientists I know have more fun than any other profession around (except, possibly, the arts, and that is a very big possibly). They have fun because the Universe has no end of surprises for them. And the best tradition of Science isallowing the universe to let you know what's going on rather than some belief or superstition or accepted theory or dogma. And if you think about it, I bet that is what you do.

Okay, sorry if I derailed this train. I felt some clarification was in order.

[ellen abbott]

I don't want to disavow this lofty perch but alas . . .

I didn't make anything up. Everything I know and do came out of books. I read the books and they were totally incomprehensible. I started anyway and as I learned different things, those things in the books became clear. Only through doing was I able to understand what I was being told. Once I learned something it became placed in the context of doing and it made perfect sense.
I'm not trying to discover this "Moldy Grail". I have one requirement - does it work better than what I do now. If it doesn't - no matter how good it might be - I won't use it.
One other factor determined what I use; availabilty. I've used materials that I preferred until they discontinued sales in this region. What I use now is also something that is so common as to be readily available. Its not made for a specific use so it has a larger customer base. I wanted a stable supply and I made it work. Thats how much thought and choice ultimately factored in.

Marc

[Tim Lewis]

If you want science why not survey/visit glass artists from all over the world to find out what they use and how they work. Then test the best mold formlas with a variety of glasses commonly used in casting and various firing schedules to see which ones work best? Then add some discussions with the engineers that make the materials and you have a real resource.

It has been done and is available on the Mixing with the Best CD. When that has been read by most people on this board we will be miles ahead of these current discussions and can discuss tweaking formulas rather than the basics.

Perhaps several people could get together to prepay someone with a UK account to avoid the hassle of dealing with currency exhcange.