difference of plaster and gypsum? translation problems (german)

[EyeofSauron]

well, im looking into making a plaster mold soon, but i have resource problems it seems.
as i am in Austria, i cant 100% translate plaster into german.

it comes up with "gips" if i translate it. the other way around if i translate "gips" to english, it comes up with gypsum, gesso and plaster.

now im not sure what the differences are and i dont wanna screw up on a sculpt.

anyone got any input?

 

[SChristides]

Hard to comment as I dont know whats local to you but I googled moulding materials when I was in a similar position of not knowing what to use and found a retailer/supplier of all things moulding. Quick phonecall and I had myself some ultracal 30. Works like plaster-of-paris but sets harder. I believe hydrocal is similar in nature.

Incidently I noticed at my local hardware store a massive bag of 'casting plaster' which contained gypsum. Tempted to buy it and try as it is cheaper.

btw. I know sweet fa when it comes to all things plaster.

 

[EyeofSauron]

i can see a problem using gypsum because its not water resistant, i.e. it would dissolve in water, or at least deteriate deteriorate. im guessing it has something in common with the stuff used for medical purposes, when you get a cast for a broken bone on your arm, leg etc. as its water resistant. im guessing this is plaster, but we also call that "gips" im seeing a problem here as we seem to not have enough words in our language to describe it right, as it translates to 3 different english terms

you can see how the translation works here xD
http://www.dict.cc/?s=gips

 

[clonesix]

Plaster is gypsum. It comes in verying degrees of harness depending on its application.

Plaster of paris is softest, and should not be used.

 

[Tigerblood]

Gypsum/Plaster of Paris is what you're looking for. Unless you're mistaking gypsum with "joint compound" which is gypsum mixed with perlite.

 

[Tigerblood]

Plaster is gypsum. It comes in verying degrees of harness depending on its application.

Plaster of paris is softest, and should not be used.

Actually Plaster of Paris is what he needs 'cause that is just a common name for gypsum.

 

[EyeofSauron]

yeah im working on a clay model that i want to cast in latex. i have sources for latex milk (liquid latex i hope), clay and tools (which is suprising as i live in narnia and nothing is ever available), and i might be lucky that the same shop has the stuff too (at least i found something that looks like this in an online catalog of different company that the shop sells too)

i dont know i just allways thought of gypsum as porous stuff that dissolves in water. maybe its just my wrong perception

 

[Tigerblood]

yeah im working on a clay model that i want to cast in latex. i have sources for latex milk (liquid latex i hope), clay and tools (which is suprising as i live in narnia and nothing is ever available), and i might be lucky that the same shop has the stuff too (at least i found something that looks like this in an online catalog of different company that the shop sells too)

i dont know i just allways thought of gypsum as porous stuff that dissolves in water. maybe its just my wrong perception

Gypsum does dissolve in water, but it's exothermic just like adding MEK-P (water) to your resin (gypsum).

 

[EyeofSauron]

well in different vids i saw it says that doing a latex cast is best to do with a plaster mold, and as liquid latex contains about 30 or so % water, i couldnt figure out how that will work, with "water on gyspum"

 

[EyeofSauron]

also im guessing making a resin cast out of a gypsum mold is just out of the question, right? at least i would think that its just to porous and would "fuse" with the resin

 

[Tigerblood]

I used it before with surfboard resin and didn't had a problem because plaster of paris get really solid. If you're in doubt, use the stuff to seal wood before molding.

 

[EyeofSauron]

its not something im gonna do multiple casts of anyway. if i model something for me, im gonna do 1 cast, and thats it more or less. and silicone molds with fibreglass shells just seem overkill, with the money you need to invest and all. i can probably do a plaster mold for like 10-20 bucks, right? investing another 20 bucks or so in liquid latex, is 40 bucks max. that seems like a good price for just doing it for me

 

[Jehuty]

Normally (when I made casts at least), you coat the inside of the mold with a release agent. Again I normally used petroleum jelly but this might differ. Anyway I don't think there's enough water in the latex to dissolve the plaster and the release agent acts as a barrier as well.

 

[Tigerblood]

Normally (when I made casts at least), you coat the inside of the mold with a release agent. Again I normally used petroleum jelly but this might differ. Anyway I don't think there's enough water in the latex to dissolve the plaster and the release agent acts as a barrier as well.

Even PAM will work.

So a lot of people wouldn't be confused with gypsum, just imagine it like using concrete (which is also exothermic just like gypsum).

 

[booksandcorsets]

Even PAM will work.

I would not use PAM as a mold release for any reasons. I've seen this go terribly wrong. Stick to Vaseline or a traditional mold release.

For a latex slush cast in a plaster/ultracal mold, no mold release is needed. You WANT it to absorb the moisture out of the latex so it would actually harm the cast.

 

[nachtinis]

Ok so some good comments here in this thread, but all in all gypsum is probably the best translation for you.

If you want more detail on it, there is a good explanation of materials if you go looking over at monstermakers.com, for example.

As for water resistance, and so forth, the main point of making plaster/gypsum/stone moulds is primarily for the ease of curing slush cast latex.

However, in the larger repeat shops where spray latex is available and time is not really an issue, for detail and longevity of the mould, fiberglass or otherwise resin moulds are used and latex is simply sprayed in in thin layers, heat cured over and over across the thin layers not letting any one layer stay dry without adding another layer of latex. Repeats of this to the desired thickness.

 

[nachtinis]

Normally (when I made casts at least), you coat the inside of the mold with a release agent. Again I normally used petroleum jelly but this might differ. Anyway I don't think there's enough water in the latex to dissolve the plaster and the release agent acts as a barrier as well.

Im not sure why this myth is going around.

Ok so plaster, in its many forms is very porous and can absorb water, acetone and just about any liquid that comes in contact with it, some liquids can bread the physical curing faster than others, but most you wont be using in a plaster mould anyway, fo rht purposes of liquid latex, you do not have to worry about this one bit.

However, water logging a plaster mould can help you break it up easier for disposal and in some cases soften it, but only a little.

Try for yourself to see what i mean, make a small plaster thing, let it cure properly, dry for a day, then submerge it in water overnight and see what happens!

If you are supremely scared of watering your plaster, you can try very fine grade concrete as well, it might not be able to capture the same detail but will work just the same at absorbing the liquids from latex applications.

YMMV

 

[tgk]

The very last thing you want to use is plaster of paris it is one of the softest of the gypsum family. The types of mould making compound that is used for moulding and casting are the harder ones such as Ultracal 30, Hydrocal, or Dental stone. All of these are formulated for extra strength, plaster of paris moulds will break apart and crumble when you are removing the sculpture from the mould. My prefered mould releases are petrolium jelly or bees wax. Good luck with your project.

 

[Tigerblood]

I would not use PAM as a mold release for any reasons. I've seen this go terribly wrong. Stick to Vaseline or a traditional mold release.

For a latex slush cast in a plaster/ultracal mold, no mold release is needed. You WANT it to absorb the moisture out of the latex so it would actually harm the cast.

I've been using it and never had a problem.

 

[nachtinis]

The problem with pam is that there are a few formulas of it and its inherent ingredients:
Canola oil, soya lecithin, natural and artificial butter flavour, annatto and propellants isobutane and propane.

The oil itself is not the greatest release to use but can work for regular rubber type moulding.

I personally have not heard of using a mould release in stone moulds for slush casting.

The primary issue with mould releases in use with stone moulds is that the porous nature of the mould is hindered when the mould release starts to clog its surface.

YMMV tho, i just havent used any mould release in any of my stone moulds.