Alginate question

How do you make an alginate mould stick to the plaster jacket? I just made a cast of an arm and the plaster kept its shape but the alginate slipped straight off and is next to impossible to relocate accurately. Any tips?

Try using Denture cream. That will hold it in place.

I've always used torn pieces of cotton batting or pieces of an old cotton towel. While the alginate is starting to set up but still wet I'll spread them over the surface, then when the plaster goes on it's got something to grab on to. I've done heads, torsos, and arms this way and they work if you're careful with it. It's easy to pull too hard and tear it off. Since I generally do my first positive in melted clay I'll use a little of that to repair alginate pulling away from the plaster, it's not perfect but it's something I'd have on hand and it works well enough to hold it to get the positive poured.

Well after a less than successful first casting I had to give the alginate process a miss this time. I think it was a combination of the fact that it was too thin and that it wouldn't hold to the jacket.
Thanks for the tips though. I need to make some test pieces for my next project so I'll give thes a go then!

Have you considered placing a couple of cube-like or rectangular (low-profile) objects over the alginate and then placing the plaster over it? It seems this would be a logical way of marking the location.

If you are only doing the arm up to the elbow, you can use a piece of pipe. Split it length wise, hot glue it back together and add a disk at the bottom. Fill it partially with alginate and stick your arm in until it sets.

Raffles said:

How do you make an alginate mould stick to the plaster jacket? I just made a cast of an arm and the plaster kept its shape but the alginate slipped straight off and is next to impossible to relocate accurately. Any tips?

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While the alginate is still in the stick soft stage you imbed cheesecloth into it being careful to tuck it into the nooks and crannies. Dont stretch it as it will deform the alginate and then brush with a thin plaster layer--paint it on quickly .Apply more cheesecloth into this plaster layer and then another layer of plaster--and continue this process until it is as thick as you want. After the first couple of layers you could use the plaster bandages also. If the alginate is already starting to set, you can slow it down by using Algi-slo. Once its set up, you pretty much cant attach the plaster shell to it, just cover it.
I recommend dental plaster as it sets more quickly and is harder. You can add Terra Alba to the mix for quicker setting, but dont get caught with a bucketful of plaster that is setting up quickly if you add this. Maybe a practice batch first to guage the setting time.

Good luck and let me know If I can help more!

Alginate is good for one off molds but more often than not it's too difficult to work with and I need multiple pieces.

I'd recommend you look into "Body-Double" by Smooth-On.
It's a two part mix silicone that can be brushed directly on the skin.
Again, like alginate it needs to be thick and backed with a mother mold of plaster.
The up side is you can cast multiple positives and if your mother mold breaks or wears out you can pop the silicone back onto the body part and make a new one.
If you are casting pieces other than from a body you could try their "Dragon Skin". I assume since you are asking about alginate though that you're taking castings from a persons body.

What was your project?

It was a partial arm cast. The reason I don't use smooth-on products is the cost. Well that and the avalibility. It may only be a few dollars more than non branded but shipping would double the costs.

To be honest I don't think I'll have to life cast anything else for a good few months so I'm sure ther'll be a comparable product to body double available locally by then.