Copyright © 1996-2006
Kevin E. Musser

Casting masonry walls in plaster

Years ago when I first became interested in architecture, I was surprised to read that first-time visitors to this country from Europe described the United States as the "land of wooden houses". Almost all residential homes in this country are framed with wood. Stone masonry, concrete and steel are used mainly in industrial structures.

The masonry walls of this sawmill under construction were
cast in hydrocal by the author using the methods described in the article.

This became clear as I began to collect photographs of structures I wanted to model. Almost all the industrial structures were of masonry construction. I decided that if I wanted to model these structures I needed an easy way to fabricate all types of masonry walls.

Over the last 10 years I have read every article I could find on the subject of molding or making masonry walls from scratch. These articles have much in common. All require a master for the mold to be formed around. Most suggest the use of mold materials that require you to be a chemical engineer in order to have access to them.

Once the mold is made you can produce hundreds, even thousands, of copies of the original.

These common steps may be fine if you are a model manufacturer, but the average modeler just wants a single copy.

Over the years I have tried countless ways of producing single molds and all but one have led either to failure or less than acceptable results. Nothing gives the depth, feel, and porous look of real masonry walls like hydrocal plaster.

Before you turn the page to the next article because you think that making walls from plaster is too difficult, read on. This method will produce a perfectly flat and square wall every time and, best of all, it is easy and inexpensive.

The main concept behind this technique is to make a mold of the wall with basswood strips on a sheet of glass over a copy of the plans.The mold is then filled with Hydrocal plaster. Let me explain it in more detail.

Materials required

Getting started. First, take a copy of the plan you are working to and tape it to a piece of window glass so that the plan shows through the glass. Lay the glass on a clean flat surface (Caution: take care with the glass as the edges may be sharp and the glass may break if pressed on too strongly). Lightly scrape the area of your work surface where the construction will take place with a razor paint scraper. This will ensure that the area is flat and free of any foreign material.

Clean the glass with glass cleaner and, when dry, lightly spray it with one-percent silicon mold release and buff it dry with a dry cloth. Mold release spray can be found at any machine shop supply. The mold release will make clean-up easier and will guarantee that the wall will not break upon removal. Select a straight length of basswood the thickness of the wall you intend to make. Make the walls at least 3/64" thick (about 9 HO scale inches), in order to reduce the risk of breakage.

Measure and cut the basswood strips and very lightly
glue them in place around the outside edge of the wall plan.

Measure and cut the basswood strips and very lightly glue them in place around the outside edge of the wall plan (see Fig. #1). If two or more walls are to be joined, be sure to compensate for the wall thickness on one of the walls by moving the basswood frame to the inside edge. Measure and glue basswood strips to the inside edge of any window or door opening. Be sure to use just enough glue to hold the wood in place and no more. Wipe off any excess glue before placing on the glass. Let the glue dry for at least 15 minutes before filling the mold.

Filling the mold. Mix enough Hydrocal plaster to fill all of the molds you have made. Make the plaster the consistency of sour cream or yogurt. If it is too watery the basswood may soak-up too much water and come loose before the plaster has hardened.

Spread the plaster in the mold using a palette knife.

Spread the plaster in the mold using a palette knife (Fig. #2). Once the mold is filled use the palette knife or straight edge and move over the mold touching the basswood edges to make the wall the proper thickness (Fig. #3) .

Once the mold is filled use the palette knife
or straight edge and move over the mold touching
the basswood edges to make the wall the proper thickness.

Work the knife in both directions and let the overflow pour over the mold (Fig. #4). This excess will clean-up after the plaster has dried, do not try to remove it now as it may disturb the mold. If there are any low spots in the plaster add a little more plaster to the location and relevel it. Work at a good pace so the plaster does not start to set up.

Work the knife in both directions and let the overflow pour over the mold. After the plaster has thoroughly dried, gently remove the basswood strips. If you did not use mold release you may have to use a small screwdriver or other small prying instrument to break the basswood free of the glass.

Once all the molds are level, let the plaster set to the consistency of soap. At this point you can gently rub the wall with the palette knife, removing any inconsistencies in the wall face. This face will become the exterior wall. Let the plaster dry for at least 24 hours before removing it from the glass.

Removing the wall. After the plaster has thoroughly dried, gently remove the basswood strips (Fig. #4). If you did not use mold release you may have to use a small screwdriver or other small prying instrument to break the basswood free of the glass.

Some remnants of the basswood and glue will remain stuck to the glass but you can clean it up with a razor paint scraper after the wall is removed. At some point during the removal of the basswood the wall should be free of the glass (Fig. #5). If you did not use the mold release you may have to give it a little nudge with your finger in order to release it.

At some point during the removal of the basswood
the wall should be free of the glass .
If you did not use the mold release you may have
to give it a little nudge with your finger in order to release it.

Final finishing. The wall is now ready for application of the stone or brick pattern. (If you are simulating a formed concrete wall no scribing is necessary.) These patterns can be scribed in with a hobby knife or small screwdriver, depending on the thickness of the mortar lines. The structure in the large photo simulates a fieldstone wall. You will notice that the edges of the walls in the photo are incomplete.

After gluing the walls together (you can use Tacky Glue or even wood glue), make a slurry of plaster and brush it on to the wall edge. Let this slurry dry for 15 minutes and gently sand the edge with 320-400 grit sandpaper. This will obscure the joint and you can re-scribe the mortar lines and paint.

That's all there is to it. This same process can also be used to make retaining walls, chimneys and bridge abutments.