I bought Type S Hydrated Lime at my local masonry builders' supply. Since this was my first time making lime putty I really wanted to go with a true quicklime or similar product, but the Type S was available, and from what I'd read it seemed as though it'd work. (note: finding plaster recipes is really hard, but if you look for fresco plaster you'll have much better luck, at least I did). So I took a 5-gallon bucket, filled it about a third of the way with water, then began adding lime and stirring until it was all well mixed. I continued to add lime and to mix well until my putty had the consistency of a thick yogurt. Then I covered the whole shebang with about an inch of water so the lime wouldn't begin to carbonate, and let it sit for a while. Funny thing about lime putty, it's actually better the longer it gets to sit, but I didn't have time to wait too long. My next batch of plaster will have great putty because I mixed it all at once and what I didn't use in our bathroom is happily sitting in 5-gallon buckets, covered with water, continuing to slake and get completely hydrated.
Getting good, sharp sand was not easy either. I ended up using general-purpose medium builders sand. Probably a little coarser than what I really needed, but so far (knocking on wood) it's holding up. I did NOT use any gypsum ("guaging plaster") because it does not hold up well in a moist environment, and let's face it: a bathroom is quite a moist environment. So we sent lime, sand, hair only.
Mixing the plaster went as follows:
- Slop out some lime putty (keep track by volume of what you're using) into a mixing tub
- Add sand: I found my recipe in a 1920s era encyclopedia. They suggested using a mix of one part putty to three parts sand for "coarse stuff," one part putty to three parts sand for the second coat, and putty alone or one part putty to one part fine sharp sand for the skim coat.
- "Knock it up" by mixing, beating, mixing, beating, mixing again and it will become more and more plasticized
- Add the hair. The amount of hair is kind of a judgement call, especially since I was not making much plaster. I added and mixed until it seemed like things were fairly "hairy" and then put some on a trowel and hit it sharply against my bucket. I ended up with about a 5-inch glob of plaster with lots of hairs visible hanging down. A fabulous, fabulous session at the Traditional Building Conference taught me that little rule of thumb...er, hair. My old encyclopedia notes that the second coat can be mixed minus the hair or with the hair in halved amounts, and straight putty or one part putty to one part sand for the final coat.
After I had what I thought was a reasonable mix, and after thoroughly wetting everything and spraying with a bonding agent, I started troweling it onto the walls. I have no shame when I say it's HARD to get plaster nice and even. HARD. Luckily, since this is the bathroom (which is the experimental room anyway and will be changed extensively at some point), I did not worry too very much. Figured if it stuck to each other and to the walls, we could sand down where necessary and make it all shake out.
All my plaster notes made mention of slopping the plaster on, then letting it sit for some time before "working it up" with a wooden or plastic float. This "working up" is apparently not my strong suit. The first time I tried it, a lot of my plaster stuck to the float. Uh-oh. Must have not let it set up long enough. The second time I tried, maybe things had been allowed to set for too long, because it didn't really feel as though I was doing anything. Time will tell, and I'm sure I'll get better at it with each new project. After all, plasterers used to apprentice for months, even years, before they were allowed to do any plasterwork in a visible location! :)