Questions about different type of plasters.

[Antonis13]

Hello all!
To be honest, there are so many things in my head about this topic and it's all kinda messed up. But I will try my best to explain it to you.

So. The last months I made 2 plaster molds, (1 half mask, and 1 full mask). And they are still alright. yesterday I realized that the plaster that I have used is plaster of paris, (In greek of course its not named that so I didn't know.). And I have heard that it is a weak plaster for molds, and it is basically just a pouring plaster. Also I was buying it 3.50 - 4 euros for 4kg plaster.

And i somehow got enlightened today, to just find and buy 25/50kg bags of plaster, and as I have seen some different types of these bags of plaster(paris, cement, building) is almost like too good to be true. Ex. 40kg of paris plaster costs 11 euros, or 25kg building plaster costs 6.90 euro. But, if I have understood correctly, is that pottery plaster is the best for molding. And the problem is that I haven't found anywhere online 25/50kg bags of pottery plaster. But I guess I will find.

So, I got some questions. What are the differences of building plaster and the other plasters? (plaster that is used in construction sites)

Could cement be used for making molds? Here is a page (of maybe the most famous company with building materials etc.) with different kind of cements, the site is in greek so translate to english. Σακευμένο Τσιμέντο

Is cement more toxic than plaster to be used inside home?

Is there a chance that pottery plaster might be more expensive than the prices that I have mentioned?

Thank you!

 

[rjenkinsgb]

Plaster of Paris can be used for fairly large items, it is considered a "casting" plaster.
40Kg for 11 euro is incredibly cheap! It costs £2 to £3 per kilo in England!


Plaster of Paris should not shrink while setting, but building plasters can shrink or crack in anything other than thin layers, if they dry out before setting hard, Some grades are very "gritty"; only the finish (surface) types are likely to have a reasonably fine powder and give a reasonable surface finish. Finish plasters are normally only used in thin layers.

There are some "Universal" or "One coat" types that can be used in thick layers and have a fine finish, but I don't know how they would act if used in large volumes for casting. It may be cheap enough just to get some and experiment?
This general type of thing:

Thistle Universal OneCoat

Finish or patch walls with a single product.

www.british-gypsum.com

Plaster and cement are not necessarily toxic, but the dust can be harmful if inhaled or it gets in your eyes; eg. building cement is strongly alkaline (caustic) and can cause eye damage rather quickly. Plaster with mineral fillers may have particles that could lodge permanently in your lungs.
Use a dust mask and eye protection with any type of plaster or cement.


Building cement (eg. Portland cement) could work if mixed with a suitable fine powder filler; I'd not try it on its own. eg. three parts of some inert filler to one part cement powder.
The problems you have with building cement would be the curing time, if you want it to reach it's maximum strength. - it has to be kept wet for several days after it appears to have set. (For serious things like building foundations, it's often kept wet for 3 - 4 weeks).

ps. The word "Cement" in English can also apply to many other things, eg. Model cement (polystyrene glue), fire cement & various others.


I'd never heard of "Pottery plaster" but that does look good, as it is intended for casting moulds with a good finish!
It looks quite similar to plaster of Paris, but even faster setting.
(25Kg costs ~ £22 here)

 

[Antonis13]

Plaster of Paris can be used for fairly large items, it is considered a "casting" plaster.
40Kg for 11 euro is incredibly cheap! It costs £2 to £3 per kilo in England!


Plaster of Paris should not shrink while setting, but building plasters can shrink or crack in anything other than thin layers, if they dry out before setting hard, Some grades are very "gritty"; only the finish (surface) types are likely to have a reasonably fine powder and give a reasonable surface finish. Finish plasters are normally only used in thin layers.

There are some "Universal" or "One coat" types that can be used in thick layers and have a fine finish, but I don't know how they would act if used in large volumes for casting. It may be cheap enough just to get some and experiment?
This general type of thing:

Thistle Universal OneCoat

Finish or patch walls with a single product.

www.british-gypsum.com

Plaster and cement are not necessarily toxic, but the dust can be harmful if inhaled or it gets in your eyes; eg. building cement is strongly alkaline (caustic) and can cause eye damage rather quickly. Plaster with mineral fillers may have particles that could lodge permanently in your lungs.
Use a dust mask and eye protection with any type of plaster or cement.


Building cement (eg. Portland cement) could work if mixed with a suitable fine powder filler; I'd not try it on its own. eg. three parts of some inert filler to one part cement powder.
The problems you have with building cement would be the curing time, if you want it to reach it's maximum strength. - it has to be kept wet for several days after it appears to have set. (For serious things like building foundations, it's often kept wet for 3 - 4 weeks).

ps. The word "Cement" in English can also apply to many other things, eg. Model cement (polystyrene glue), fire cement & various others.


I'd never heard of "Pottery plaster" but that does look good, as it is intended for casting moulds with a good finish!
It looks quite similar to plaster of Paris, but even faster setting.
(25Kg costs ~ £22 here)

Thank you very much for your long reply!!!

I've managed to found online pottery plaster and it is about the same price for it's weight, 25kg 23.5 euros. I might get it once to see what's up.

 

[udog]

Pottery plaster is a good plaster for casting in anything that needs absorption to dry such as pottery clay pours and latex. Ideal for latex.
Builder plaster, there is a bit of confusion depending on how it's called where you are. Here there is a finer version that I understand would be "plaster"(escayola in spanish), and there is a rougher one that I understand would be "gypsum" (yeso in spanish)not sure). The first works for molding (have used it for years, specially for waste molds but also for lastex, surface will last less), weaker than stone plaster but works ok.
You also have stone plaster and dental plaster to achieve stronger and harder molds.
As for cement, wouldn´t recommend it for molding.