WTB: Three cheap preferably round shield blanks FOR SCIENCE

[Russ Mitchell]

re:
viewtopic.php?f=4&t=131492&p=1946136#p1946136

The only thing holding me back from doing a re-do on my previous work with Tarsusi's recipe is that I don't have molds. Ceramic products for the purpose are inappropriate (too expensive, wrong shape, etc). But if somebody can do up three roughly-identical bowl-type round shield blanks for a reasonable price, I'm in the market. Strapping, weight, etcetera, are irrelevant, as these will never see any sort of combat.

[Gregoire de Lyon]

Would used plastic saucer sleds work? I might be able to find some now that it is end of season...

[Russ Mitchell]

They might, if they've a nice uniform bowl shape. (That's why ceramics are out -- flowerpot bottoms and the like are never made this way, but with all kinds of weird ridges and stuff that the hide will catch, pull, and deform on, as I've found out the hard way repeatedly). In some ways they might even be superior, given that the hide's not going to stick to it.

[Kel Rekuta]

How about cheap chinese woks? Utterly enormous ones are available a lot cheaper than one could get such a shape made by a potter. Line it with plastic dropcloth et voila! No chance of cracking those things in a press.

I had planned to turn a set of molds out of soft maple but the plastic sled idea is a simple solution.... hmmm.

[Russ Mitchell]

That's an idea, too -- have to check out the asian grocery down the street. Though I'm still envious of these guys just for being able to create a bowl shape with their hides that doesn't have excess material making ugly triangles!

[Swamp Stick]

I'm a potter. I can tell you that getting anything non local (in reference to pottery) is going to be cost prohibitive, shipping will be $$$$. That is if the potter has a kiln big enough to handle the diameter you want to start with. I don't know what diameter you need. I would use a plaster mold anyway, it's easier. It won't shrink on drying etc. I don't know if you need a convex or concave mold (read further down, same principle). Get a flat surface that can hold your mold. Attach a dowel or metal rod in the center of where your mold will be, but do it in a way that it can be removed. I've used a screw from underneath. The dowel should be the exact height of the finished mold. Now cut a piece of Plexiglas the negative shape of the bowl along the radius of the bowl. Now attach to the top of the dowel (the dowel will now be dead center). Attach it in a way that the plexi can rotate 360* on top of the dowel. The smoother the edge of the plexi is the less smoothing you might have to do later. Coat dowel with a release agent (mold soap etc.). Mix up plaster and let it setup to where it is like mortar/drywall mud. Trowel it around the dowel and rotate the plexi around to knock off the high spots. Repeat until you have a nice smooth mold. It is best to make just one batch of plaster, but I've done multiple batches. I just make sure the the "skin" coat is continuous. For your purpose I'm not sure it would make a difference. Remove the plexi. You can do a little smoothing with a wet sponge when the plaster sets up a bit more, it has to to be "just right" so you don't make it worse though. Let cure and remove dowel (now you find out if your release agent is worth a damm). Fill hole with plaster. Smooth out with drywall screen or sand paper if needed. That gives you a nice convex mold. Do the same but make the plexi shape the other way (convex) add a retaining wall to get a concave form. If you want to get fancy make the opposite of what you want (convex/concave) and the use that mold to cast your working molds off of. They will all be exactly the same that way. Do a search online to learn about release agents and plaster to water ratios etc. Make sure to seal the mold well, plaster will absorb glue, fresh plaster, or anything else that will stick to it. Don't end up with a "sculpture". You can use "filler" like crumpled newspaper or damp sand covered in plastic to take up space if the mold needs to be really thick, make sure it can support the weight of the plaster though. Also make sure the mold can support itself and the plaster is not so thin that it breaks. I tend to prefer massive molds if I will use them more than once, I always end up using too much filler and make a weak spot.

I hope that made sense. Best of luck.

Bjorn

[Swamp Stick]

Oh, make your plexi as rigid as possible, you will distort the shape if it bends while scraping.

[Russ Mitchell]

I'd have to re-read that four or five times -- I've never made a mold, and completely failed to visualize the process.
(Did three semesters of ceramics as an elective in college, so I hear you on shipping and prohibitive cost, or I'd have begged kiln access of somebody and dove in myself.)

[Cet]

I think I have some round aluminum blanks kicking around- about 18" in dia . If they'd work e-mail me. ldr_gto@verizon.net

[Swamp Stick]

You know how they scree the top of a concrete sidewalk? Same thing. Attach end of shaped plexi "scree" on top of dowel in a way that it can rotate in a circle. The space under the rotating scree will be the shape of the mold. Put plaster around dowel and scree off excess by rotating plexi in a circle. Like centering a piece of clay where you cut off the uncentered parts. Lot simpler than it sounds. Get a copy of Mold Making for the Potter by Firth. My copy came with a handy volume and water/plaster calculator. This is a method to make jigger/jolly molds along with other "round object" molds.

Bjorn

[Russ Mitchell]

@Cet: email sent from my work addy
@Swamp: thanks, that makes quite a bit more sense.

[InsaneIrish]

Russ, have you thought about carving a positive mold out of plaster?

Easy to carve, easy to re-add material, and pretty cheap.

[Russ Mitchell]

Never even heard of one, heh. I'll google it.

[InsaneIrish]

Basically build up a plaster of paris block in roughly a larger size than you need. Make sure to wait until to dries completely. Then build a profile template out of lumber. Look at the diagram below.

After you are done, seal the entire positive mold a polyurethane sealer.

You should end up with a pretty good mold.

[Russ Mitchell]

ooh, neat. There's all KINDS of cool stuff that could be done that way if the plaster's cheap enough...