properties of mold materials

[Lauri Levanto]

Frit casting open face reliefs, I use ceramic molds with
kiln wash. That works OK.

When casting deeper, solid objects I have used a mold mix
of plaster, kaolin, grogg or luto. The wall thickness
is about 1 ½". Cracking is a problem.

The molds seem to survive the ramp up. Somewhere
above 700 C cracks emerge. I can keep them in control
with a chicken wire mesh baked into the mold, but
the crack marks are seen on the surface of the piece.

Once I succeeded to take advantage of this crackeled
feature, but in most pieces the crack lines are disturbing.

What are the benefits of plaster-silica mold
compared to plaster-kaolin-grogg?

-lauri

[John Kurman]

I use nothing but plaster-silica, and, if respectully treated, it behaves quite well. I suspect that the cracks are there already and they become noticeable at 700C or so. Item, a year or two ago, I was in a rush and cured a mold at 400F straight from casting, and it cracked like hell. The cracks formed on the interior surface of the casting. Water. Steam. Gotta be it. That's my story and I'm sticking to it.

Now, if I'm in a hurry, I dry the mold in the kiln at 200F for 24 - 48 hours. Works so far.

[Bert Weiss]

Lauri

With finesse, you should be able to develop a mold formula that does not crack. The cracks come from water not finding pathways out of the plaster mix. The best way that I know to create those pathways is to include some materials of varying particle size.

I was taught plaster/silica equal parts by volume. Then I was told that it would crack. What was up with that?

I later learned that making a formula by weight is much more accurate and pertinent to success. Keeping plaster to about 40% by weight is the first requirement. Having the right amount of water is also critical. Too much water makes the mix easy to work with, but results in a weak mold.

Grog will stick to glass so having it on the surface is problematic. The solution is to make a face coat without any grog. Then a stiff backup mix with grog in it. Luto is good. Kaolin can be good, but not too much. diatomaceous earth can be good. Fiberglass strands can be good.

It is most important to allow heat to pass beneath the mold. Mold on the kiln floor is not a good thing.

[charlie holden]

There are as many different mold formulas as there are casters. I think you need silica because you need something that sticks together as it gets hotter. This may be part of the defenition of refractory, I don't know. But everybody says you need a refractory material in your mold.

Bert is right about giving the water a path out of the interior of the mold. One option is to make the mold porous by adding a fiber that whill burn out. Some people throw in some chopped up newspaper. Another option is cellulose used for filtering by beer makers.

I recently saw a video of a presentation by Anne Robinson. She used a mix of one kilo each of plaster, silica and grog with one liter of water and a handful of chopped paper on her face coat. She wasn't clear whether she uses the same mix on outer coats or not, other than that she adds some chopped fiber glass for strength. There may be more about her method on the recent CD from the UK on kiln casting. I haven't been able to buy a copy yet. I'm pretty sure she casts lead crystal from Gaffer though. So her mix might not hold up to the higher temps needed for soda-lime glasses.

I don't think wire helps unless you need something to keep the mold together if there's a total failure. The wire expands more than the mold does as it heats up.

Incidentally, Ted Sawyer recently pointed out that there are both soda-lime and borosilicate fiber glasses out there. If you are putting chopped soda-lime fiber glass in your molds then it is melting right along with your cast glass and not adding any strength at all. Do a test of your chopped fiber by taking a small pile of it to fusing temp and see if it is still intact when you're done.

[Gale aka artistefem]

Bert...........can the molds be placed flat on kiln shelves that are sitting on short legs in the bottom of a side firing kiln?

Or should the molds be slightly elevated on kiln furniture even above the shelf?

Thanx

[Bert Weiss]

Bert...........can the molds be placed flat on kiln shelves that are sitting on short legs in the bottom of a side firing kiln?

Or should the molds be slightly elevated on kiln furniture even above the shelf?

Thanx

Gale

I think that a kiln shelf raised up a bit is fine. I had a great deal of trouble working on the floor. I am advised that the problem was the there was not enough of a way for heat to get in and out of the base of the mold. These were large open faced bas relief molds.

It has also been suggested to me that a bed of sand might just do the trick.

[Gale aka artistefem]

Thanks Bert........I've been setting my molds directly on my shelves with no apparent problem.

Just thought there might be some arcane heat and air movement knowledge that I was not factoring into to my castwork firing schedules - LOL!

[ellen abbott]

Lauri

I have used several different mold materials over the years. I think almost anything will work as far as mold materials.

There are as many different mold formulas as there are casters.

There is more because most casters know more than one. You need to find a firing schedule for your mold. It may take longer at different temperatures. Now we use a one part plaster/two parts silica and 44-46 parts water. This formula doesn't need to soak at some called for temperatures. However, it does need to soak at different temperatures.

It was all trial and error to find a good efficient kiln schedule for the mold.

If your mold making is easy or efficient (or any reason to keep doing it) then change your firing to suit your mold. If its not then try some other stuff. Eventually, your mold and your firing will have to come together.

marc (Ellen's tech guy)

[Bert Weiss]

Thanks Bert........I've been setting my molds directly on my shelves with no apparent problem.

Just thought there might be some arcane heat and air movement knowledge that I was not factoring into to my castwork firing schedules - LOL!

Whatever works. I was having trouble with bubbles that would blow when the glass got to the bubble viscosity. The catch was that the mold hadn't let go of all it's gas by the time the glass got hot. These were long slow firings. I was advised that raising up the shelves might help. I gave up on the project before working out all the bugs. It was time to cut losses and do something that worked.

The frustrating thing about this project for me now, is that, I think I could make good castings using new glasses and a few other tricks. I don't have running water or a reasonable place to make a plaster shop in my present shop. In my old house, I had a basement that I did the plaster thing in. My house now has a 4' sandy basement. Not exactly well suited to a plaster shop.

I was doing a collaborative project with a woodcarver who would send me rubber positives to cast with. He is really good and very successful at his art. I had this great big slab that I tried several times to cast and never did get a good one. I got some smaller ones.

[charlie holden]

At GAS, Olympic Color Rods was showing a mold made with a product called Castalot. They had pulled nine castings from the mold and it still looked like it was in good shape. The glass from the ninth casting was clean and had strong detail. They said that they use a thin layer of kiln wash on it and put small vent holes into all the low points.

Bullseye was showing boxes made with vermiculite board that they screw together and cast into. They put plaster/silica shapes into the boxes, pour a thin layer of plaster/silica into the bottom to hold the shapes in place, line the walls with fiber paper and fire. The vermiculite boards survived intact and could be unscrewed to release the glass then screwed back together and fired into again. The vermiculite board is commonly used in fireproof doors. It can be tooled just like wood.

ch

[Nikki ONeill]

Charlie: I didn't go to GAS but Ted Sawyer gave a workshop at Vitrum where we used the the vermiculite mold material. Bullseye just came out with a tech sheet on it. I don't know who's going to market it, but it's inexpersive and you can buy it by the palate directly from the US distributor in Charlotte NC. The company is Skamol, and it's made in Denmark. They were nice enough to send me one sheet to test. It does carve like wood but it is a bit more crumbly. Hence you also have to be careful unscrewing and re-screwing the pieces together so as not to strip the threads. Neat stuff! What did you think of the Castalot material? It's hard to believe that after several firings it doesn't develop cracks. Pretty expensive, too. Have you or anyone here used it in sculptural castings?
Nikki, in Hot, sunny MD

[Elizabeth in UK]

Hi
I found a UK site detailing the Scamolex vermiculite stuff
http://www.woodstoves.co.uk/skamol.htm
Seems the panels come with textures - do you think this would work for slumping?
Elizabeth in UK
http://www.stainedglass.co.uk

[Nikki ONeill]

I don't see why it wouldn't work for casting or slumping, as long as it was lined with release material. In Ted's workshop, we lined the sides and bottom of the box with 1/8" fiber paper, so the glass never touched the vermiculite. Check the manufacturer's site (Skamol, Denmark) for further information. There are different compressive strengths available. The US rep. said that the one that seems most popular to glass people is the second one down from the most compressed. (forget which number that is). PLease share what you find out if you're going to experiment. Seems like a product with great possibilities for casters and slumpers.
Nikki

[Lauri Levanto]

Thanks for all the tips,

I'll try plaster-silica as soon as I can get silica sand.

For the innermost layer I use one part plaster, 2 parts
Aluminnium oxide. That gives a satine surface to the glass.
If possible to reach, I gently rub the surface with a soft cloth
and the vacuum the dust off.
The coat must be very thin.

I have understood that the chrystal structure of plaster changes around 700 C, and I have blaimed that for cracking.
It seems to contract very much at that temp. It sounds possible that it releases gasses or steam at that point.

Mica boxes are not an option, as I want to cast sculpture
in a waste mold.

-lauri

[Brian and Jenny Blanthorn]

Frit casting open face reliefs, I use ceramic molds with
kiln wash. That works OK.

When casting deeper, solid objects I have used a mold mix
of plaster, kaolin, grogg or luto. The wall thickness
is about 1 ½". Cracking is a problem.

The molds seem to survive the ramp up. Somewhere
above 700 C cracks emerge. I can keep them in control
with a chicken wire mesh baked into the mold, but
the crack marks are seen on the surface of the piece.

Once I succeeded to take advantage of this crackeled
feature, but in most pieces the crack lines are disturbing.

What are the benefits of plaster-silica mold
compared to plaster-kaolin-grogg?

-lauri

I would expect kaolin / china clay n shuch things 2 shrink on first firing

Silica would not

Grog which is fired clay would not

[ellen abbott]

A couple of things in this thread about mold materials caught my eye.

First, all our molds are binder/refractory (plaster/silica). The refractory is stuff that won't burn up. Silica - at the temperatures we cast - stays the same through the firing. Its a pile of sand at the beginning and its a pile of sand at the end.
Plaster is different. When you add plaster to water it undergoes a chemical reaction that causes a crystalline structure to form. The density of this structure is determined by the amount of water. More water = larger crystals = less density. Drying plaster actually makes it stronger. Firing plaster drives off the chemically bound water. As the water is driven off the larger crystals break down due to incomplete structure.
Everything expands when hot. It seems simple but its really not. Expansion
causes stress. Just like glass, if you heat your mold too fast or unevenly it will break. Yes, at certain temperatures the plaster and the silica shrink. This is above the softening point of the glass and has the effect of hugging the glass tightly. Great for fine detail. That part should not affect your firing of glass if the previous run up is done right.
So, Mix your plaster and silica for pre-firing (green) strength and for strength after firing. Detailed and fragile surfaces need to come out of the kiln like a pile of sand.
A couple of things will make stronger more survivable molds. Denser plaster. Several plasters use 70 parts water per 100 parts. There are special purpose plasters that use considerably less. I use Hydrocal white and 45 parts water. Less water to remove and smaller tighter crystalline structure. You could also add refractory cement. Like plaster it is chemically bound but its strength is many times greater and it is meant to keep its form after the water is removed.
One other thing. Kaolin and grog and anything else clay-like are different forms of silica. Some may have an alumina content. Lauri was that aluminium oxide or alumina hydrate?
I know this is kind of glossed over. Consider it a basic of the basics.

[charlie holden]

I'm afraid I'm going to disagree with a couple of things here. They don't change the overall ideas of how to mix your materials. I'm just a stickler for details and I went and looked some of this stuff up yesterday. I'm really just trying to work out some pictures in my head of how all these things in the mix effect each other.

>First, all our molds are binder/refractory (plaster/silica). The refractory is stuff that won't burn up. Silica - at the temperatures we cast - stays the same through the firing. Its a pile of sand at the beginning and its a pile of sand at the end. <

Silica does change during the firing. At about 1060 F it goes through quartz inversion and actually expands suddenly about 2%. It's important to try to move slowly through this temperature to let the mold heat up evenly, so that parts of your mold aren't expanding when other parts are not. That's one thing that can lead to cracks.

Silica also does become reactive at fusing temperatures which can be exacerbated when it is in contact with glass. The fluxes in the glass can make the silica more reactive. In other words, silica that is not surounded
by plaster or seperated from the glass by kiln wash will stick to the glass. Alumina hydrate and zirconia, alternative, more expensive refractories, will not.

>Everything expands when hot. It seems simple but its really not. Expansion causes stress. Just like glass, if you heat your mold too fast or unevenly it will break. Yes, at certain temperatures the plaster and the silica shrink. This is above the softening point of the glass and has the effect of hugging the glass tightly. Great for fine detail. That part should not affect your firing of glass if the previous run up is done right.<

According to Kervin and Fenton, although the silica becomes reactive, it is still still somehow stable. The plaster crystals begin to lose strength as they heat up and break down, but the silica, (or whatever refractory) give them something to bind onto. This reduces their contraction and increases the strength of the mold.

I'm not sure I understand what you're saying about the mold hugging the glass as the plaster/silica shrinks, unless you are talking about the cool down back through quartz inversion. What I picture in my head is that the individual crystals of plaster, long, interlinked needles, are falling apart. But I don't think that leads to the mold contracting as a whole. I may be wrong.

Something else we haven't gotten into, that Kervin and Fenton write about, is mixing in varying sizes of particles, either of refractory or modifiers like grog, so that the particles pack tighter together. I think this gives the plaster more sturture for each crystal to bind around.

[ellen abbott]

I'm not going to argue with anything Kervin and Fenton say (at least I hope not).
I usually hold my molds at 1100°. This is slightly above the inversion temp and I consider that part of the drying sequence.
When you cast glass there are four distinct parts of the firing.
1. Firing the mold
2. Firing the glass
3. Annealing
4. Cooling to room temperature.
Each one is a seperate encyclopedia. This was all about number one.
Silica reacting is number two. Its just as complicated. And I'm kind of a stickler for details too. The silica doesn't start to react with the glass until 1500°. Before then the glass is reacting to the plaster.
Alumina hydrate will stick to glass. Common kiln wash is 50/50 kaolin/alumina hydrate.
Particle size is bullshit. If you want fine detail you have to use fine particles.

[Lauri Levanto]

Ellen,
I have used plaster and alumina oxide as splash layer,
also tested plaster and BE blue kiln wash.

-lauri

[ellen abbott]

Lauri
I checked a few references and heres what I think. Aluminum oxide in Europe is hydrated alumina over here. Hydrated alumina and EPK (kaolin) are combined to make common kiln wash. I have used that in a lot of molds either, 50/50, or sometimes with talc added. It does the same thing as silica. Neither better nor worse.
BTW, over here aluminum oxide is used as a sandlasting abrasive - its what I use.
marc