Best Mold material

Heather Nelson · Post by **Heather Nelson** » Tue Sep 23, 2003 6:03 pm

I need to make a slumping mold for a piece of glass 1 1/2 " in depth x 8 " long x 1 3/4 wide. It looks like the top of a butter dish with a flat rim of approx. 1/8" all the way around.

I've done some reading on different mold making materials and casting materials.

I am now looking for anyone elses opinion on the best material to use.

I will need to make 50-100 of these pieces so I need something that will last for several firings..

Thanks for your help.

Heather

Tom White · Post by **Tom White** » Wed Sep 24, 2003 6:37 am

The shape you describe does not lend itself well to glass slumping. In normal slumping procedure a glass blank is heated on top of the mold shape until it softens and conforms to the shape of the mold below it. The deeper the mold the more the change in size of the slumped item from the original blank. For example a 12" circle slumped into a rather deep slumping mold may end up only 6 or 8 inches in diameter. The 12 inches of glass is still there but in the form of down one side of the mold, across the bottom and up the other side of the mold. The shape you desire is 1.5" down + 1.75" across + 1.5" up =4.5". You might be able to produce this shape by using a 6mm thick blank and stretching (rather than slumping) it into the mold. In other words doing a drop out firing into a mold rather than falling free to the shelf below. The sides of the mold would surely need some draft to facilitate removing the finished item from the mold. Your blank would have to be 3/4" to 1" wider than the mold cavity on all sides to avoid the edge pulling into the mold and dropping down the side of the mold. The 1/8" rim could be cut on a tile saw after forming the item. If fire polishing of the cut edge is needed return the item to the mold for this operation.

Best wishes,
Tom in Texas

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 7:07 am

Morning Tom,

So basically you are saying this technique will not work. I was hoping to get a plastic blank and use this for making a mold. Then trying my luck at using an oversize glass to slump into this mold. The piece is not square cornered in shape. It is more like a tent shape on the ends.(if you look from one end to the other)
The rim just needs to be cut and smoothed to break all sharp edges. This will be covered with a gasket material.

It sounds like what you are telling me is that slumping does not necessarily take the shape of the slumping mold..
Is that correct?

What type of casting material would you use?

Thanks for your help..

rosanna gusler · Post by **rosanna gusler** » Wed Sep 24, 2003 7:17 am

heather i think you could make it work. i would cast a plaster hump mold of my shape. i would use this hump mold to form several paper clay slump molds with an oversized rim. use a 1/4" slab of paperclay. then you would be able to use an oversized blank and get your shape. i have used this method alot and it is easier than it sounds. what are you making? a light fixture? msnosey.

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 7:41 am

Thanks for the idea. Sorry for the stupidity here but I think you are saying instead of slumping in a slump mold let the glass drape over a hump mold. (Slump ,hump diddley dump)
Or am I way off?

What kind of mold material would you use?

These are fume hoods for an Antique Oiler..

Thanks,

Heather

rosanna gusler · Post by **rosanna gusler** » Wed Sep 24, 2003 7:53 am

i was clear as mud. i would make a plaster hump mold. the clay goes over this and takes its shape. the glass is fired in the clay (slump) mold. there are shrinkage considerations with clay. depends on how tight your size tolerances are on your finished product. e- me if you want . rosanna

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 7:54 am

What an idiot I am . I realized after I sent the last reply you gave me the mold material to use. How does this material work? Where can I get this?

Thank you

Tom White · Post by **Tom White** » Wed Sep 24, 2003 8:25 am

Heather, what I was trying to say is that slumping glass does not normally involve stretching the glass. I had envisoned a rectangular cross section rather than the triangular shape you describe in your second post. I still think this form will involve stretching the glass to some extent. The shape of the mold lends itself to easy removal of the formed item. Paperclay as Roseanna suggested or any ceramic clay formed over a positive model of your shape snd fired to bisque should produce a durable mold. If dimensions are critical the model might have to be slightly oversize to allow for shrinkage of the mold in the bisque firing. Some type of release coating will be needed to prevent the glass from sticking to the mold in the slump firing.

It sounds like what you are telling me is that slumping does not necessarily take the shape of the slumping mold..
Is that correct?

Quite the opposite. I was saying that the glass softens and DOES conform to the shape of the slumping mold. In my example a 12" diameter circle of glass placed on a 12 1/4" diameter x 4" deep slumping mold would produce a bowl shape that was the exact shape of the bottom of that mold but it would not be 12" in diameter or 4" deep when slumped into that mold. The center of the glass blank will soften and deform downward first. As this happens the edge of the blank is pulled down the inside of the mold until the glass in the center touches the bottom of the mold. The glass then settles into the shape of the mold from the center outward and up the sides of the mold. The measurement of the bottom of the glass bowl from one edge, down the side, across the bottom and up the other side to the far edge is still 12". The difference is that instead of a flat sheet 12" in diameter the glass has been shaped into a bowl shape (by slumping) that is less than 12" wide and less than 4" deep. The deeper the mold in relation to its diameter the the larger the variation in size of the slumped item from the size of the mold. A shallow (12" dia x 1" deep) plate mold would produce a plate near 11" in diameter when a 12" blank is slumped into it.

Best wishes,
Tom in Texas

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 8:32 am

Thanks Tom, I understand what you are saying. Thank you so much for the help. Do you have suggestions on where I can purchase the mold material?
Heather

Tom White · Post by **Tom White** » Wed Sep 24, 2003 8:50 am

Most any ceramic supply house or local ceramic studio should have ceramic clay available. I am used to working with low fire ( Cone 04-06 ) earthenware clay. Clay without grog will be smoother than clay with grog in it. Paperclay is ceramic clay with paper fibers blended into it. It is a newer material and might not be as available at local studios. I do not sell the paperclay, only the white cone 04-06 clay without grog in my business. Use about 1/4" thickness for your mold and dry it completely before firing it to bisque. Make your clay mold with a flat bottom to sit level on the kiln shelf and with a rim of 1" on all sides to support the glass. Small vent holes may be needed to let the air under the glass escape as the glass fills the mold. Drill them before firing the mold to bisque.

Best wishes,
Tom in Texas

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 9:06 am

Thank you very much for all you help.....

rosanna gusler · Post by **rosanna gusler** » Wed Sep 24, 2003 11:20 am

it is very much worth the trouble to get the paper clay. it warps way less when drying. axner pottery sells it. i love this stuff. rosanna

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 12:17 pm

Rosanna,
I looked on the Axner web-site and they had several different materials for making molds. I called them to get info on which one to buy and they said "they can't give me any information but have a book they will sell me" Can you tell me which material I should buy for glass mold making? I really appreciate your help!! Thank you.
Heather

Nikki ONeill · Post by **Nikki ONeill** » Wed Sep 24, 2003 12:42 pm

Rosanna:
I didn't know you used paper clay. It seems to be a really smooth material for making molds. Have you had any problems with it? The people at Axner were very nice and sent me two free samples, one of which is the low-fire white paper clay. I just received them last night, and if we ever get our power back, I'm like to run some tests this weekend. Do you have their book, and if so is it worth while buying? I'm interested in making slumping molds and also flat relief negatives for subsequent castings.
Camping out is starting to get old. "they say" we may get power back this Saturday. That's 8 days!
Nikki

Heather Nelson · Post by **Heather Nelson** » Wed Sep 24, 2003 12:53 pm

Sorry to hear about your power problems..

It must not have been my lucky day speaking with Axner. I had a young lady on the phone that new nothing about mold making material for glass and she said they had a book I could buy that would help...
I thanked her and thought I would try Rosanna again..

Hope you get your power back soon...

Heather

rosanna gusler · Post by **rosanna gusler** » Wed Sep 24, 2003 1:33 pm

heather, just ask for low fire paper clay. white. nikki, those thrown molds i showed you this winter were paper clay. i also have made some slab formed deep bowl molds, drop rings and a couple different olive server type molds. you are going to have to come down sometime and play. rosanna