Mixing plaster of paris

[richhotrain]

rrebell

Funny, many times the instructions are just wrong. 

Is that really the case?  I can speak for WS Light Hydrocal. The instructions for mixing are precise and accurate.

Rich

[selector]

Only one quick contribution from me, based on experience:

Yes, use soapy water to coat the inside of the mould before you pour the mixture into it.

Also, consider light jiggling, even lifting and dropping short distances (3-4 mm, tapping) to get air to lift and move up to the surface once you have the pour in place.  This is not an easy thing to do, for obvious reasons, but it can greatly reduce the friable nature of the product when the material cures.

[Overmod]

Lastspikemike

Often it can help reduce cracking if you introduce fiber into the plaster.

This is good but I'd recommend using a skim coat in the mold first and then introducing the fiber-loaded material behind it.  This was SOP (in reverse) for high-quality wall plaster in the old horsehair days, and now for AVG-style shotcrete fabrication of inside walls without furring and rocking...

I confess to having made regular use of acrylic bonding agents in plaster casting for architectural purposes; it allows for thinner slip and stronger intricate details while also bonding nicely to the 'composite' material away from the face.

[BATMAN]

http://www.observationsblog.com/sciencetechnologyexperiments/plaster-of-paris-mixing-guide 

 

[maxman]

selector

Also, consider light jiggling, even lifting and dropping short distances

I had one experience to do some plaster moulds when I belonged to a club.  I did the jiggling and dropping thing and can attest to the fact that the air came out.

Unfortunately that did nothing to help the plaster.

[trainnut1250]

Macman44

 My box of plaster still seems fresh enough, so no immediate concerns there.

I'll also give Lightweight Hydrocal a try when I next buy.

Paul.

 

 

Paul,

(From someone who has mixed 100's of batches of plaster).

The symptoms you describe are from old plaster. It should "go off" (set up) with a definitive heat build up and should become pretty hard. Soft, crumbly castings generally mean the plaster is bad.  Plaster of Paris is softer than hydrocal but still sets pretty hard. If the box has been around for awhile it likely has absorbed water and gone bad.

Experiment with some measuring containers to get a good ratio of water to plaster and then use the same ratio everytime. It makes the process much easier. I mix my plaster to the "soft yogurt" thickness for rock castings.

I dont reccomend WS Light Hydrocal. It is very expensive and I dont like the finished product compared to "regular hydrocal" you buy at the plaster and lath supply store.

 

Have fun,

 

Guy