Retarding plaster set

[Terry Gallentine]

I have been working with low relief plaster molds to texture glass and I need to retard the working time of the plaster so I can scratch and manipulate the plaster. I have found information where others have used either citric acid (lemon juice) or tartaric acid (cream of tartar) to achieve this. Do any of you folks have any experience with these techniques?

[Morganica]

What plasters are you using, how much manipulation are we talking about, how long after demolding? I carve into the plaster for pieces like this and have about a 4-hour working window after demold:

The two pieces on the bottom (this is straight out of the kiln, without coldworking) are carved after cleaning out the mold, to get the swirly line texture. I'm using plain old pottery plaster/silica flour in a 60:40 ratio (sorta--there are additives), demolding and cleaning it up, then going into do the texture carving.

I've tried retarding the plaster set by putting maybe a quarter-cup of vinegar per half-gallon of water (or so), and then chilling the mixture to well below room temp. It increased the set time but I didn't clock exactly how much--probably 30-40 minutes, but the mold seemed weaker as well. I only did it the one time, didn't think it worth the trouble.

One thing about carving into a refractory mold; it works well for straight plaster/silica mixes but in my experience it can cut through the smooth "skin" that some mold mixes form, down to a sandier layer below that can embed into your glass and be a pain to remove.

[Terry Gallentine]

I am not doing a plaster slab and then carving it. I am laying down a thin layer (1/8th inch or so) of plaster on top of a kiln shelf and then raking it or pressing it to create a texture in the wet plaster. After it sets, I spray a thin layer of kiln wash on it and allow it to finish hardening. I put it in the kiln to heat out most of the remaining moisture and then I place glass on it and fire it to temps that allow it to pick up texture. The plaster layer ends up with small cracks and separates from the kiln shelf but this doesn't seem to have an effect on the irregular textures that I am going for. A delayed set would help me with working time on the plaster. I will experiment with citric acid to see how it works but any experiences out there would be appreciated.

Thanks

[Jim Murphy]

Hi Terry,

You may simply use 'cold water' [60ºF or cooler] for your plaster mix to give extra set time
rather than risk some of the negative effects of added acid retarders, i.e., brittleness, more cracking, glass surface
imperfections, etc.

Good luck!

Jim

[Bert Weiss]

I suggest trying dry plaster. No water. The working time is quite long depending on relative humidity, lol, and it will take impressions. It won't crack with the heat and takes no drying time.

I usually work with a mixture of other refractory materials. 75% sand and 25% alumina hydrate. The first recipe I was given was for 80% sand and 20% dry plaster. I know others who do it with just the plaster. The addition of some larger particles can create a stronger matrix of particles. Each recipe has it's ups and downs. Just try it.

[Terry Gallentine]

I have experimented with some sand and plaster mixes for dry casting but I needed something that would yield minimal textures on a very flat piece of glass. I went ahead and tried mixing 5% lemon juice in my water before I mixed in the dry plaster and gave it a try. What it yielded so far after drying for 2 hours is a plaster that seems to work somewhat like soft clay. I went ahead and textured the plaster layer. I am going to leave it over night to see how it sets up to a harder state (hopefully it will) that will be conducive to adding some carved work. So far so good.

[Morganica]

Ahhh. More complete information is always helpful. Why are you spraying kilnwash on the plaster? If it's refractory it shouldn't need kilnwash at all.

What plaster are you using? Are you prewetting your kilnshelf before applying the plaster? How long do you allow the plaster to set before you start the drying process?

There are better ways to retard plaster set than using citric acid (actually sodium citrate...or any acid, for that matter). The problem with acidifying plaster is that it's easy to go too far so that the plaster won't set well at all, and it almost always results in a weaker slab with more cracks.

Other ways:
--Use very cold water, as Jim and I mentioned. This one is the most reliable for retarding the set without affecting strength.
--Don't allow a slake and only lightly mix--the longer it takes for the plaster to fully absorb water, the more you'll slow the set.
--Prewet your shelf--part of what's happening to speed the set, most likely, is the kilnshelf absorbing water from the plaster. If you soak the shelf with water before you lay in the plaster you'll slow that down.
--Reduce the ratio of plaster:other ingredients in your mix.
--Keep the water:plaster ratio higher. This will also considerably weaken the mix, and increase the cracking, but it also slows things down.
--I've never tried this one, but a potter friend swears by it--pour a can of stale beer into the water before mixing up your plaster. If you try it, let me know how it works.

[Peter Angel]

There are many commercially available plaster retarders eg:

http://www.usg.com/usg-plaster-retarders.html

The only concern is the effect they might have on the mold, on the glass and they might be toxic at kiln temperatures.

P.S I saw some girls at art school adding some Red Bull to plaster and it slowed the setting time right down!

[Terry Gallentine]

Ahhh Morganica,
I apologize for my incomplete information and will try to answer your questions better.
I am spraying kiln wash (strong on alumina hydrate) on the plaster because I have used kiln wash on plaster molds for 30 years now. In the beginning I found that I would get some plaster stick on the glass if I didn't.
I am using #1 pottery plaster.
I am not wetting the shelf before laying down the plaster.
The water to plaster ratio is low to create a stronger casting.
All that said, I used the addition of the citric acid and got the following results.
2 1/2 hours after I poured the plaster, it was semi soft and I could rake it and make impressions in it.
After setting overnight (12 hours) it was hard enough for carving.

As of this morning everything seems to be working well. I still have to do a minimal cure firing on them before I fire some glass on them. I expect some cracking to occur because of the expansion difference between the shelf and the plaster but as I said before some cracking is acceptable.
Thanks again, I'll let you know how it works out.

[Lauri Levanto]

At our sculpture glass skimmed milk was used, also diluted PVA glue.
I have no personal experience with these.
-lauri