Bad Plaster Tool Pour

I had a bit of a setback today (November 19, 2001). I poured the dental plaster backing for the tile grinding tool, and some of the tiles floated up far enough as to render the tool useless. I'm going to have to buy some new tiles and pour again, and try not to repeat the mistake.

(Note: I've been corrected by Richard Schwartz on the ATM list. The stuff I poured is properly called dental stone, not plaster.)

Here's how I prepared the plaster tool. Maybe you can suggest how I can do this better so I don't ruin my tool.

First, I wiped a thin layer of petroleum jelly on the face of my mirror blank. The mirror has previously been rough ground nearly to the desired radius with a 2 1/4" pipe fitting. Then I measured out a 30" sheet of aluminum foil, and folded it lengthwise in thirds, carefully.

Then I taped one end of the foil to the edge of the mirror so it was even with the bottom, and my wife helped me as I carefully wrapped the foil dam around the mirror. I stretched it tight then taped it in place with masking tape. I think heavy paper would work better than foil but I didn't have any handy.

Then I tore off enough plastic kitchen wrap to cover the face of the mirror and go up and over the sides of the foil dam. I carefully stretched the plastic so it stuck to the petroleum jelly without any big wrinkles. It didn't reach over the top on its sides to I taped it in place (so the plaster wouldn't run down the side of the mirror), and generally tried to keep wrinkles out of the plastic on the inside.

Then my wife mixed up the plaster for me in a paper paint bucket. The box it was shipped in had a label that said to use 30 ml of water for each 110 grams of plaster. But I had neither a scale nor graduate cylinder. I would suggest purchasing a postal scale and if you're in a backward country like the U.S.A., converting the liquid measurements into cups and using a kitchen measuring cup.

My wife had been taught to pour plaster an interesting way - she poured some water into the bucket, then started pouring plaster in slowly, scooping it with a cup, and stirring it in with her hands. She understood that the point to stop was when the plaster stopped sinking into the water but instead formed an island that wouldn't sink. At that point she stirred up the plaster thoroughly with her hand and then I poured it into the mold. It was about 2 1/2" thick. My wife vibrated the board the mirror was resting on to encourage the bubbles to float to the surface.

I didn't recall until long after pouring that some people put wire mesh into the middle of the plaster to reinforce it. It happens my wife bought some gutter guard today, and she suggested I could have used a short piece of that. But I didn't think to mention it until after the pour.

We weren't at all sure we had the proportion of plaster to water right, and it looked really thin so I was anxious while it set up. It took an awful long time to set, I think because it was really cold in the garage and we'd used very cold water from the outdoor tap. We had poured the extra plaster in a plastic bag and periodically I would squeeze it to check.

While I stared expectantly my wife put the second coat of paint on my workbench.

Eventually it did harden up. After the pour the plaster had settled a bit and a layer of clear water was left on top. Eventually even that water was absorbed, and the plaster got warm. Chunks of plaster from the waste bag were pretty hard but at first could be easily broken.

I had set the mirror and mold on a small board for the pour and carried them in the house so they'd cure overnight in a warm place. But after a while I got impatient to see the results of our work so I peeled off the mold.

The mirror wouldn't come off at first, because in places a little plaster had gone down to overlap the side of the mirror. I chipped it off and then the mirror slid off easily. And then my disappointment - two thirds of the tiles were covered with plaster.

I thought it was just a thin layer at first, so I tried to scrape it off with a table knife, and when that failed, chiseling it with a spoon. My wife let me know I shouldn't use table utensils to chisel plaster, so I got a sharp knife and started scoring the plaster surface. Then my wife suggested she chisel it off with a putty knife, and after a while of that she asked me to get the hammer.

I put a sheet of plastic on the kitchen floor and then my wife wailed away at the plaster on top of the tiles for a while. She expressed concern that my tiles were uneven, but I reassurred her that I could soon grind the tiles away until they were even and then it wouldn't matter. But after she had completely uncovered a few tiles it became apparent that the tool was ruined - the ones at the edge of the plaster covering were sharply tilted, going as much as 1/4" in under the plaster. There was no way I could use this tool.

So tomorrow I'm going to go to a nearby tile store and see if they have some tile sheet like the stuff Dan Cassarro sent me. I think what I may do to avoid this is to skip the plastic wrap over the mirror, and put more petroleum jelly on it, so the tiles are stuck to the petroleum jelly. It's possible that when my wife was vibrating the mold it knocked the titles up off the face of the mirror. I'm not real sure. Fortunately Dan sent me lots of plaster.