Furniture Detective: Do-it-yourself plaster casting

You don’t have to be a master to cast a little plaster


featuredImage
Materials to cast plaster molds, such as plaster of Paris and common baby powder, are easy to find and not all that difficult to master with a little practice. Using clay to make a mold costs less and gives the craftsman more control. Photo courtesy Fred Taylor.

Several years ago a very nice mirror was given to me by a friend who was moving away. The mirror is a three pane reproduction Empire wall mirror approximately 65 inches wide by 27 inches high, a 1905 copy of an 1830 original (the glass is dated, as is all 20th century mirror glass but that’s another story). The frame was marginally loose, which was easily repaired and the gold leaf and underlying gesso were in acceptable condition after a little cleaning and touch up.

The only real problem with the piece were the four 3 1/2 inch corner blocks with the applied swirling sunburst decoration – only three of the four had the applique and it was terribly obvious that something was missing. The choice was to remove all the remaining pieces or find a way to make and install a reasonable facsimile of the sunburst.

Since my carving skills are restricted to Thanksgiving that certainly wasn’t an option. The most obvious choice seemed to be to make a mold from one of the existing decorations and pour a new applique. Sounded simple enough and in the end it turned out to be – but it took a while to get there.

Not really knowing where to start I went to a well known art supply store, complete with the requisite number of body-decorated, shampoo-deprived young and not-so-young very helpful employees. There I received an absolute avalanche of information about firing temperatures for hard casts, what to do about sagging molds, lost wax casting techniques (where does the wax go?), how to tint the casts, how to cool them, how to do everything I needed to know about casting except how to make a small plaster or gesso sunburst for my mirror.

After deciding that I wasn’t going to be casting a Rodin or Remington I decided to revert to the old tried and true KISS (Keep It Simple Sister) method and see what I came up with. I bought one pound of the cheapest modeling-clay-like material that I could find. It was called “Plasteline” and cost me about $1 for a pound, roughly a 3-inch cube. The art supply people told me it absolutely, positively would not work so I was further reassured. They insisted I buy a substance called “Sculpey II” for $1.25 for two ounces. I passed. Then I went to the hardware store and bought a pound of plain old plaster of Paris in a cardboard box for about another buck. So now I’m into this project for about $2 and change: that’s more my speed.

There were some starts and stops, several trials and many errors but in the end it did work and here’s how:

Start with a piece of clay roughly 20 percent wider than the piece you are going to cast and about twice as thick and work the clay between your hands until it is warm and soft. Next dust a small, clean (very clean) paint brush with baby powder and spread the powder on the piece you plan to copy for the mold. The powder keeps the clay from sticking to the positive. Don’t apply the powder directly from the shaker to the piece because you will get too much on your positive cast.

If the positive you are about to duplicate is still attached or otherwise inconvenient, as in the case of the sunburst, just press your lump of clay directly over the piece. Use a flat piece of wood dusted with baby powder that is bigger than your clay to apply even pressure. The flat block will also put a flat back on the clay mold so that it will sit level after you remove it and turn it over. If the positive is a loose piece such a small medallion or patera, lay the clay on a flat board (dusted with powder) and press the positive into the clay, again using a powdered block of wood to insure even pressure and a flat back. Next, remove the clay mold or the loose positive very slowly and carefully.

Don’t worry too much about distortion of the clay as you remove it because you can reshape it with your hands from the outside. Some molds are better than others and often you can’t see the difference until the end when you have to start over. Inspect your molds as you release them and if they don’t look clean and clear your casts won’t be either. Just reform the clay and make another mold.

Once your molds are made and laid out on a flat surface, it’s time to mix plaster. Follow the directions if you like or just mix the stuff slowly with water to about the consistency of pancake mix but without the lumps. You can experiment with the consistency. Pour the mixed plaster into your molds until they are full to overflowing. Don’t worry about having too much, the excess is easily removed when dry. It’s worse to not fill up a mold and end up with a concave back after the plaster dries. Now use a toothpick to help distribute the plaster within the mold since air can get trapped in the pockets and details. Visualize the mold to help release air pockets and insure a full cast.

Now comes the hard part: waiting. The directions on the plaster will say that it sets up in two to four hours, which it does, but usually not hard enough to withstand pulling it from a mold, especially if your cast has a lot of fine detail. If you can stand it, allow the plaster to cure overnight before releasing the mold. When you are ready, gently start to remove the mold by working at the edges and corners with your fingers, use no tools if absolutely possible. The soft clay should come off in your hands leaving a nice white  plaster cast of your original. If you are releasing a loose positive from a block of clay be careful that you don’t break your original. Patience is the key here. Your new cast may have excess plaster around the edges, which is easily removed with sandpaper or an emery board. If it has some of the clay mold stuck in it, gently remove it with a knifepoint or other sharp object.

Since you probably are going to apply your new cast to a flat surface, the back must be as perfectly flat as you can make it or it may crack when you glue it down. Turn the cast over and use a sharp rasp to flatten the back, finishing it off with 80-grit sandpaper wrapped on a block of wood. You don’t want it much smoother than that for glue adhesion. When you are ready to apply your new applique, use some 5-minute epoxy and light pressure from a C-clamp or other clamp arrangement. The plaster cast will accept stain well, although it may go a little dark; so try your color on some scrap material and stain the piece before you glue it since the epoxy will not take a stain if you happen to get a little on the piece during the glue process. The new plaster will also accept most surface finishes such as paint, lacquer and urethanes.

All of the foregoing sounds simple enough and it really is, but be prepared for mistakes and leave yourself time to learn as you employ and enjoy your new skill.

Send your comments, questions and pictures to Fred Taylor, P.O. Box 215, Crystal River, FL 34423 or info@furnituredetective.com. Visit Fred’s Web site: www.furnituredetective.com.

His book “How To Be a Furniture Detective” is available for $18.95 plus $3 S&H. Also available is Fred and Gail Taylor’s DVD, “Identification of Older & Antique Furniture,” ($17 + $3 S&H) and a bound compilation of the first 60 columns of “Common Sense Antiques by Fred Taylor” ($25 + $3 S&H). For more information call 800-387-6377, fax 352-563-2916 or info@furnituredetective.com.



MORE RESOURCES FOR ANTIQUES ENTHUSIASTS

*Great Books, CDs, Price Guides & More
*Share YOUR Thoughts in the Antique Trader Forums
*Check out our FREE Online Classified Ads
*Sign up for your FREE AntiqueTrader.com e-mail newsletter
*Visit the Antique Trader blog for the latest news and views from the Antique Trader editors
*Enter the Antique Trader Treasure Hunt Sweepstakes

Leave a Reply