What makes 30 minute plaster of paris set in 12 minutes?

[darth9x9]

I bought a bag of plaster of paris from Home Depot yesterday. It clearly says that the working time is 20 minutes and that it sets in 30 minutes. But every batch I mix, it sets in 12 minutes. Does anybody know why this happens?

[mustanggt]

I'm no expert on plaster, but did you use the right amount of water in the mixture?

[Anonymous]

There was a thred on "Mixing Plaster of Paris" a short while ago. The one thing that may have an impact on your mix is water temperature, the warmer to hotter the water the faster the chemical reaction.

HTH
Will

[oleirish]

That is why I don't use plaster of paris ,It sets to fast,I've herd that you can re-tard the sitting time with MILK never tryed that yet! but might work,OH also cut back on the water and mixing time,OK now I've walked all around your [?]May be its an old bag??I agree with WILL try ice water![^]
JIM

[joeyegarner]

I had some problems with that, what I done is add water to my mixture. It seemed to work for me. The most important thing is to be ready to go to work when you get it mixed. Joey

[darth9x9]

Yeah, I know the colder the water, the slower the set. Also, adding white vinegar slows the setting time. I even intentionally mixed a runny batch and it still set in 14 minutes. I would think when the bag says you have 20 minutes of working time, that I should have 20 minutes from the instant I add water to start mixing. Something else is going on here. Hopefully a chemist will read this and let us know what the deal is.

[cjcrescent]

Bill;

I'm no chemist but I do know that what minerals/chemical additives (chlorine,fluride etc), thats been added to tap water can affect plaster setting time. Try this experiment. When mixing, don't use anything but your hands to mix it with and get a gallon of distilled water and try it. If the setting time doesn't change, I'd have to say the plaster is old.

While I was working the OR before I retired, we could always tell when our plaster casting material was getting old. Instead of setting in the 10-12 minutes for the regular casting material it would set up in less than five. Not enough time to mold the cast to the limb we were applying it to.

[cacole]

Another thing you can experiment with is vinegar -- I have read somewhere that vinegar will retard the setting of plaster. How much vinegar to use, I have no idea. And I assume they meant distilled white vinegar.

[darth9x9]

QUOTE: Originally posted by cacole

-- I have read somewhere that vinegar will retard the setting of plaster.

Uh, yeah, you probably read it in my post two entries above....or maybe you didn't.......

BC

[darth9x9]

QUOTE: Originally posted by cjcrescent

Bill;

I'm no chemist but I do know that what minerals/chemical additives (chlorine,fluride etc), thats been added to tap water can affect plaster setting time. Try this experiment. When mixing, don't use anything but your hands to mix it with and get a gallon of distilled water and try it. If the setting time doesn't change, I'd have to say the plaster is old.

While I was working the OR before I retired, we could always tell when our plaster casting material was getting old. Instead of setting in the 10-12 minutes for the regular casting material it would set up in less than five. Not enough time to mold the cast to the limb we were applying it to.

I am on well water, mixing in a plastic bowl, using a metal spoon. I will keep experimenting to find out what the cause is.

BC

[rexhea]

About a teaspoon to a tablespoon of any kind of vinegar is all you need to retard the setting time. I have even used apple vinegar when I had nothing else. Also, as cjcrescent said, mixing with your finger tips works as well. I tried this after seeing it done in a MR DVD and was surprised at the results. Distilled water is better than tap particularly if your water is "hard" or "soft" by mineral content. Note: I use plaster of paris and even though I buy some time with these methods, you still have to work fast with only about 5 minute of ideal time.

REX

REX

[claycts]

QUOTE: Originally posted by darth9x9

I bought a bag of plaster of paris from Home Depot yesterday. It clearly says that the working time is 20 minutes and that it sets in 30 minutes. But every batch I mix, it sets in 12 minutes. Does anybody know why this happens?

In the DVD just sent my MRM this is the key:
Cold water and vinegar in a bucket, POUR in only as match as you will beed of the Plaster DO NOT STIR it adds air bunnels which SPEEDS the drying time. OBSERVE the plaster is it breaks down into the water mxi. When it is mostley dissolve use your HANDS to nead the mix, DO NOT STIR. YOu can then break up the chunks. I add a little Brown oxide to my mix so I do not have SNOW!
Hope this helps!
George P.

[Anonymous]

George: I trust every thing went well on Thursday. Good to see you "up" and about as it were.
Will

[rexhea]

George, thanks for being more specific with this method on the DVD. I forgot about letting the dry plaster absorb the water first. At first I was very skeptical about this, but it did improve the working time. The kneading of the plaster with the fingers not only seems to help the working time, but also keeps air bubbles from forming as bad.

FYI: I have never got the amount of working time that is printed on the instructions.

REX

[cwclark]

one thing that makes it set faster is if the tub you are using to mix it in is used over and over again...it must be cleaned thoughly before a new batch is made or it will set up in no time at all especially in plastic tubs...the plaster gets impregnated into the plastic causing it to set faster...another thing to do is add a bit of vinegar..the vinegar will retard the set up time....chuck

[Anonymous]

Dried plaster of paris can act as a catalyst with wet plaster to cause it to set up faster. This can be good or bad, depending on what you're trying to do.

Want it to set faster? Add some dried plaster that's mashed up.

Want it to dry slower? Use cold water and a touch of vinegar.

Mark in Utah

[claycts]

QUOTE: Originally posted by Overdurff

George: I trust every thing went well on Thursday. Good to see you "up" and about as it were.
Will

Yes, I survived, but was in limbo with the forum locked.
Back on this subject. Solved my plater problem:
1. Bought a GLASS mixing bowl at Doller store
2. White Vinegar
3. Put water in the frig
4. added the plaster to the water and vinegar
5. Used hands
6. 35 min. work time
Take care
George P.

[rexhea]

Wow! George, that is a good working time. One thing to watch out for is adding too much vinegar. I have experienced a weaker, more powdery plaster if I have added more vinegar than necessary.

REX [:)]

[selector]

Good to hear you're back on your feet, as it were, George. Just can't keep a good man down. [:D]