Sharp cuts in Clay? (Picture)

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Hello everyone,

for my current project I need to make "holes" in form of cuts into the clay.
They also have to be sharp, no rounded edges.
Here an example:


How could I accomplish such a feat?
What tools would I need (all my tools up to this point are makeshift from stuff around the house)

On another note:
How does poured Silicone react to insulation foam?
The clay on my project is not very thick on some parts of the project, if I make such "holes" I will expose insulation foam.
It's common for people to make their own tools. Honestly, you could just go to Michael's or some other craft store. They'll have a few different assortments of clay tool kits. They're usually either sold as wood tools or metal tools and since you want really clean cuts, go for the metal. Might be packs of 5 or 10 tools together, shouldn't run you more than 10 bucks. Could always try an xacto blade or kitchen knife first too.

No idea on the second problem, sorry lol.
Thanks for the reply

maybe the colours of my picture are too bad to be a good example - sorry.
of course to only "cut" into clay a knife is the obvious solution but my problem is I need a "gab" or "hole"
wich means I have to somehow cut the shape with a knife and then somehow get the clay out of there without deform the "border" or "edge" of those cuts (the hole)

Its the getting the cleay out of there I have a problem with.
Haha :D

Well I would love me some weird shaped cuts like in the picture, no stars or hearts from cookie cutters :)

Even if - that would not work either because a cookie cutter only cuts "the border" (which I can do by hand with a knive in any form I wish)
But since we have a lot of clay here, the clay is still connected on the BOTTOM with more clay.
Which means I somehow have to carve the clay out without damaging the border.

urgh...I hope my explanation makes sence...maybe looking at the picture again does.
Sorry for any weird english :/
maybe this will help

see the cracks go deep in, which means the clay has te be carved out.
but since the cracks are very thin, that seems like an IMPOSSIBLE task
Going back to middle school art here and using the rubbish paper clay stuff.

I would tackle the problem by using tools like those, something with a sharp angle that can "drag" the clay out, or using a knife to scratch out the cut.

Just remember little to no clay work experience here, just observations and thoughts.
Yes those tools are the first thing that comes to mind on that problem right?

You said "drag" the clay out wich is interesting and might actually work.

if you have a larger area you want to cut down those tools are perfect but if you want to dig something like cracks - like me - with pointy ends - not even a millimeter thick I wouldnt know what to do.
But if I can actually "drag" the clay from the corners out by scratching on thicker parts that would be creat!

I guess I have to buy that stuff then
I can help with your chemistry question. Insulation foam comes in two flavors, styrene and urethane. Styrene/polystyrene (Styrofoam, or Owens Corning pink or blue insulation board, commonly carved for large projects like motorcycle seats, prop casts, stuff like that) melts when exposed to pretty much any aromatic hydrocarbon. That means anything you'd find on the back of a can of spraypaint, or sounds like you'd find it there. "Petroleum Distillates" is usually a good indicator that you might find a light aromatic hydrocarbon inside.

Urethane foam is a little more resilient, but not by much. You still want to avoid light hydrocarbons, but urethane foams like Great Stuff or DAP that come in a spray can offer slightly improved resistance against heavier hydrocarbons such as oils.

It boils down to what the solvent is in your silicone.
Many of the silicone products I've used in the past have used acetic acid as their solvent, and trace amounts of platinum as the curing catalyst.
If your silicone smells like vinegar when you open it up, chances are good you're using a similar type, and that it will NOT react adversely to the insulation foam.

If you look up the MSDS for the product and examine it for contents, look for things like butane, heptane, xylene, toluene, methyl ethyl ketones (or any ketone- or acetone-type solvent. or frankly, anything with a methylene base, ethylene bases aren't AS aggressive but may still pit or weaken your foam, I THINK).

Silicone itself is a hydrocarbon, but a largely inert one, so what you're looking for are the chemicals mixed into it to get it to flow or harden quickly.

As far as the cuts go, what I would recommend trying, is making a tool like the ones you see above, with a hard steel wire. Make a loop the size you want, grip the loose ends with a pair of Vise-Grips, and peel away the cut a little bit at a time because if you try to do too much the clay will bend the wire.

If you use a coarser wire you will need to grind or file the leading edge to some kind of angle so that it actually cuts instead of just mooshing. FIner wires may not work so well unless they're very, very stiff.
WOW Redtail

Thank you VERY much for your reply. Very helpfull!

Those "holes" of mine will be holes in the final cast so the Silicone does not need to make a perfect imprint or even stick to the foam, as long as it sticks to the clay around it (the "wall" of the hole if you will)
my only fear was that it will.
1. melt - so the silicone just gets lost inside my very large object, wich means I will waste a LOT of Silicone.
2. It wont harden, even around the clay - and therefore will become useless and a waste of a lot of money.

The Silicone itself is a big problem though.
I moved to Japan and cannot find Silicone here that seems to be fitting!

Sadly I personally dont know anything about the data and what it has to say (actually my first time sculpting at all)
But someone said to me as long as the Shore Hardness and the Elongation at Break of the Japanese Silicone is near
the rebound 25 from Smooth-on - it should be fine.

There are 2 Problems:
1. for whatever reason, there is no such information written on the Silicone cans in the stores.
Even the Store clerks couldnt find out by 1. looking on the producers web-page 2. phoning the producer and ask.
Seriously no intel whatsoever...
2. The few companies for wich I found information have Silicone with:

Shore Hardness TypeA 47
Elongation at Break 170%

Far from the number of rebound 25.
Also the 2 Compounds of the Japanese Silicone are 1 Can (maybe 2 Liters) and one TINY bottle (maybe 30ml)
mixing this will kill me - someone who doesnt know anything.
Do you have a cordless drill?
Go to a hardware store with a gardening supply section and ask for a tulip bulb planter. It will come in the form of a small screw-auger fitting that you can put in your drill's chuck. Usually it's used for drilling holes in dirt but you can use it very well as a paint stirrer, roofing tar stirrer, light-duty repair cement mixer, BOnd-O mixer, et-cetera. THey make larger paint stirrer attachments for this as well, but they're bigger and more expensive and generally meant for attaching to a large AC-powered corded drill and very rapidly mixing huge quantities of the stuff, so you might find that even if it fits in the can, you'll have the stuff all over your walls in seconds.

If you use the bulb planter, run it in the reverse setting to churn the mixture downward instead of upward.

I wish I could help more with the silicone, I usually use plaster of paris or sand to cast things, myself. I haven't got much experience casting.

If you can, try to mail-order the stuff you need, but before you buy, look online for what's called an MSDS, or Material Safety Data Sheet. This is public domain for just about every consumer product manufactured, and lists anything from flammability, radiochemical concerns, to toxicology and reactivity. It will also list any dangerous or sensitive compounds contained in the product and how to handle them safely.
Hmm, sorry I didn't see about the holes part. I think it would depend on how far you need to go. I would think you could just make two parallel, angled cuts so that they essentially make a wedge in the clay. To make it easier you could make the first cut straight- perpendicular to the clay you're working with, and make the second cut angled a bit so you're cutting that bit of clay from the source. And then to get the slice of clay out, like Vandoore said, drag it out with uh, you could just get a piece of wire or something and hook it. I have tools just like the ones in the picture that I've modified myself. I wouldn't recommend them for such a precise and clean cut as the edges are not beveled and they're not very sharp. If I have some time tomorrow I'll try and experiment myself, how deep does it have to be?
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