how to support glass for this mold?

[seachange]

Hi

A long time ago I saw these glass bowls, and fell in love with the shape. Did contact the artist in case he/she wanted to share where to get the mold, but reply was that it was a found object.

oval-shape.jpg (20.57 KiB) Viewed 14617 times

Haven't been lucky to find something as nice as those, so thought would try shape below:

Lined it with aluminium foil to see how big the glass piece has to be (a trick I had read on this board). As you can see in this picture, the glass should be quite a bit longer and wider than the mold itself.

It is the first time I am attempting something like this, and can't quite figure out how to support the glass so that as it bends it falls into the mold and somehow works out all right.

Is it very tricky to do? Is this done with kilnwashed posts?

If yes, advice on how high above the mold would be great. Does anyone has a picture of a similar set up? The pointy ends of the mold are about 1" higher than the lowest part of the side curves.

Don't really expect the first one to be great, just to avoid a big disaster while I am learning

Many thanks, seachange

[Bob]

Hi Seachange,

Lovely shape. Here are a couple of suggestions:

1) start out using float glass to get the glass size prior to slumping right. Don't worry about annealing them ... they are tests only.
2) The glass will stretch while it slumps so for a first pass I would cut the glass about 1/4 inch longer on each end and about 1/2 wider at the widest point for each side.
It may draw in at the central part where it is not supported... just guessing. Using this approach the edge of the finished bowl will be slightly inside the edge of the mould.
3) If the glass overlaps the two ends of the mould... and if you position it very carefully so it is balanced then that is all the support you should need.
4) heat it slowly ( 300f/hr ) until it starts to bend and then hold it there so that it slumps very slowly. It may take 30 to 45 minutes for it to slump into the mould. If it looks like it won't slump in that time increase the temperature 25F .You'll have to peak in the kiln to see how the piece is slumping so be careful to wear natural fibre clothing... cotton... eye protection ... all the safety concerns.

You might have to tweak the shape of the window glass glank a bit but once you are happy then do the same with you art glass and adjust the schedules .

Best of luck.

Cheers,

Bob

[Geo]

I thought it was always better to drape over stainless steel molds rather than slump. Am I incorrect?

[Bert Weiss]

I thought it was always better to drape over stainless steel molds rather than slump. Am I incorrect?

Geo, in my experience, draping over yields shapes I don't care for. Slumping in is better. The reason for the "rule" you state is that stainless steel shrinks more than glass when cooling. If the shape is sufficiently drafted, the glass will simply ride up, and be fine. I have spherical shaped ss molds that, if cooled all the way, to room temp, will grab on to the glass. The remedy is to turn it upside down and reheat until the glass falls out. Usually, I get to the molds while they are still warm and the glass does come out on it's own.

In my experience, I have never had glass break when slumping in to stainless. I have had glass break when draping over ceramic. Again, if the shape is appropriate, this can also be done without mishap.

My suggestion to Seachange is to simply trace the top of the mold and place that shape of glass on it. You could get away making it the width of a marker pen bigger. The glass will fall in and be smaller. I have seen edges hang up, when you overhang them. That is tricky.

There are techniques out there using multiple molds for deep slumps. You start out with a large shallower mold, and slump in. Then the next step can be a deeper mold, etc.

[Bob]

Hi Bert,

Good points. The reason I suggested the blank be slightly large than the mould is that once the piece is slumped the edge will be at or inside the mould. You get the largest glass piece possible for the shape of the mould. If the slump is at the lowest temperature possible then the thin rim of glass that extends outside the mould will not actually drape bend down over the edge of the mould . Instead as the glass slumps into the mould the glass rim outside the mould pops up. The edge of the mould is the center of a teeter totter. I just want to clarify why I suggested the extra rim of glass. It will be a bit tricky to get the right width for the lowest part of the rim. That is why I suggest the tests with float glass first.

I guess the bigger question is... does size matter?

Cheers,

Bob

[Morganica]

I think whether or not the glass will hang over the outside edge depends on the thickness of the glass, the type/stiffness of glass you're using, the schedule AND the degree of overhang. As a rule of thumb, when working with fusible glasses I can usually extend the glass past the mold by approximately the thickness of the blank at the edges. So if my mold rim is 10 inches across and the blank is 1/4 inch, I shouldn't have any problem making the blank 10.25 inches. The center glass will start sagging and pulling the glass down into the mold before the edges are soften enough to overcome surface tension and move. (or at least that's my theory)

If I need the slump to fill the entire mold, especially if the rim is fairly small in proportion to the volume then I make the larger blank and slump it using a collar. I get a piece of fiber board, flip the mold upside down on the board, trace the rim and cut out the shape, then kilnwash the top of the collar and the inside edge really well.

Then I set the mold on the kilnshelf. If I collar a slumping mold with a symmetrical, flat rim, I can usually fit the collar snugly enough that it stays up without kiln furniture. If not, I prop the collar up on kiln furniture, so that the top edge is even or a bit higher with the tallest point of the rim. It doesn't have to be an exact fit; essentially, I'm making a drop-out ring but it falls into the mold instead of onto the kilnshelf. Then all I do is center the blank on the collar and slump.

The slump into/onto rule is mostly a myth--what you really need to pay attention to is making sure you don't trap the glass in the mold.

[Mike Jordan]

If you are looking for the original shape, do some googling for banana split bowls or ice cream boats. Most are glass but there are a number of different shapes in stainless steel and some in china and ceramic that might be able to take the heat of slumping into. There are some shapes that are like your picture and others that are equally neat shapes.

Mike

[seachange]

Hi Morganica,

Have 2 questions.

a) When you support the extra large blank on the fiber board, up to how much extra border does this system allows you to add?
For example, with this mold, the alu foil pattern shows that I have to cut the glass 3/4" all around larger than the bowl edges, to end up with glass bowl similar to mold size.
Therefore, if the fiber board hole is cut to exact mold size, I will have a 3/4" border resting on it.

Will this 3/4" wide border slide down, or is there a risk that it might grab on the fiber board?
Should I cut the fiber board hole a bit bigger than the mold, so that only about 1/4 inch of the glass rests on it?

b) How do you shape the inside edge of the fiber board drop out ring?
Like this /__ (angled towards the board), or like this \__ (angled towards the hole). or vertical l___ ?

Many thanks, seachange

[seachange]

Many thanks for all the replies, they all make sense to me.

Ideally I'd like to get the glass bowl about the same size as the metal mold (as Bob asks, in this case size does matter ). Because of the narrow width compared to the depth, kept thinking that if I cut the glass just to size the resulting bowl will be rather small.

Remembered reading in the (now lost) archives, something about supporting the glass on kiln furniture on the very edge of both the glass and the kiln post, so that as it starts slumping it slides - hopefully gracefully and evenly - off the supports and into the mold.

The "even" part has always concerned me, Morganica's explanation of support all around seems a lot safer to me.

After reading all your suggestions, the action plan is to start as Bob suggested with float. Will have to vary the firing schedule for the final piece, but it will be a beginning. Have to learn to cut ovals at the same time, so waste glass is a very good idea.

Cut glass to the size of the alu foil, and support it with kilnwashed fiber board above the mold, as Morganica says.

Use coffin kiln so I can look from the top and center everything properly.

Use slow schedule and peep often.

Hope I am not forgetting anything important

Many thanks, seachange

[seachange]

Many thanks Mike, I will certainly look into that. Have been somewhat obsessed with the original shape in the color photo for a couple of years.

Didn't have a kiln big enough then, now it is all slowly coming together.

2013 is starting great

Many thanks again, seachange

[Laurie Spray]

It would be easy enough to make a clay slab and alter it on this stainless steel shape to make the shape you like........Just make real sure the bottom is flat or cut it out and make it "Bottomless"!!

[rosanna gusler]

It would be easy enough to make a clay slab and alter it on this stainless steel shape to make the shape you like........Just make real sure the bottom is flat or cut it out and make it "Bottomless"!!

best solution yet. rosanna

[seachange]

Hi Laurie,

Many thanks for the suggestion - by the way I really like your bottomless molds. The problem currently is that I don't have a proper studio space to work with clay. It takes me far too much time to unpack all my clay working tools (rollers, board, etc etc etc), then clean up and pack away again...it is just not worth while at present for one mold.

Yesterday I found an antique bowl in ebay UK that is very close to the original, with the extended bits at each end. Will pass this to a friend that works with clay, to make a bigger one modeled on it, so that I can get a good size glass bowl easily. This will have to wait for our winter, when we all have a bit of time to play.

However, will still try Morganica's method on my current mold. I think it is a very useful technique to have under one's belt, specially for single pieces...plus I like a challenge

All best wishes, seachange

[Morganica]

Hi Morganica,

Have 2 questions.

a) When you support the extra large blank on the fiber board, up to how much extra border does this system allows you to add?
For example, with this mold, the alu foil pattern shows that I have to cut the glass 3/4" all around larger than the bowl edges, to end up with glass bowl similar to mold size.
Therefore, if the fiber board hole is cut to exact mold size, I will have a 3/4" border resting on it.

Will this 3/4" wide border slide down, or is there a risk that it might grab on the fiber board?
Should I cut the fiber board hole a bit bigger than the mold, so that only about 1/4 inch of the glass rests on it?

b) How do you shape the inside edge of the fiber board drop out ring?
Like this /__ (angled towards the board), or like this \__ (angled towards the hole). or vertical l___ ?

Many thanks, seachange

Collar size? I've never found a limit, as long as the kiln will hold it. If you're expecting all the glass on the collar to head down the "hole" and fill the mold, though, there's a practical limit of an inch or so. More than that, and it's easier for the inside glass to slump and stretch than for the glass to be dragged across a flat surface down into the mold. And the more glass you have on the collar, the more coldworking you'll be doing to clean up the edges.

I don't worry about making the fit exact, as long as there's less than an eighth inch gap between the mold and the collar. The collar is at or slightly above the mold, so as long as gravity's working you don't really need it to fit well. Ditto for shaping the inside edge--I tend to cut it so it's slightly wider at the top than the bottom, but I don't think you need to.

[seachange]

Hi Morganica

many thanks for your replies, I will be trying this.

Best regards, seachange