Scratch Building a Stone Arch Viaduct
– Casting and Construction
text and photos by Phil Bonzon, MMR |
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 Okay, It happened to me again; I wanted something that
did not exist commercially and having scratchbuilt many
times before when the need arrived. I said why not? A stone
arch viaduct for my B&O and one for the Gateway Division’s
switching layout was what I wanted, only plaster castings
was not something that I usually make. The last time, I was
a Cub Scout and made a plaster casting of an Indian’s head,
which I still have. At the 2010 MCoR Convention, Marty
Vaughn, MMR (and MCoR AP Chairman) gave a clinic on castings
and that was just what I needed to push me forward.
First I made decided to use plaster castings that would be
6" long and join at the keystone that is at the center of
the arch. This way, I could make the bridges to any length,
as long as it was a multiple of 6", and the parts would join
at the narrowest part of the casting, meaning less things to
match up.
Next, I made a sketch of the part and decided to carve the
master pattern out of homosote, because it carved easily,
had desired thickness and some texture to the surface. After
carving with an X-Acto #11 blade, I coated the pattern with
shellac, let it dry, set the pattern on wax paper and
sprayed it with PAM, cooking oil, to serve as a release
agent.
The latex rubber mold was made from “Castin’ Craft’s Mold
Builder” that I got at Hobby Lobby. Using a cheap disposable
brush, I applied about twenty thin coats to the pattern;
letting each coat dry before applying the next. To speed up
the drying process, between coats, I put the pattern/mold in
the oven for 30 minutes at 160 degrees.
After the latex mold had cured, I placed in a box of river
sand, to support the mold while pouring the plaster.
I wet the mold with water that had a little detergent in it,
then mixed Patching Plaster to a consistency of a milk shake
and carefully poured the plaster into the mold, so that it
filled the mold to the top. After curing, I popped out the
casting and repeated the process until all the castings
needed were made.
For the inner structure of the viaduct, I ripped a wood 1x4
to the desired width for the roadbed and the pier supports.
After the castings were completely dry, I trimmed up the
edges with a file and used a combination of hot glue and
yellow construction glue to bond the castings and wood
together. I found that if the castings are still moist the
hot glue will not provide a temporary bond. The yellow glue
provides the permanent bond.
To fill in the underside of the arches, I mixed a stiff
mixture (DQ Blizzard stiff) of patching plaster and applied
it with a narrow putty knife. After drying, I used a wood
rasp and course sandpaper to bring it to its final shape and
finish. Also, I used these to clean up any misalignment at
the keystone.
Using a razor saw, I connected the ‘stone joints’ from one
side to the other and carved in the joints with a X-Acto #11
blade. Also, using a #11 blade, I recarved the joints at the
keystones.
Using Woodland Scenics’ acrylic washes and their
instructions for coloring rock castings; I colored the
viaduct. The base was made from ¼" Masonite; landscaped with
foam board insulation covered with plaster cloth; a mixture
of patching plaster and sand; blended tuff, ground foliage;
sand; twigs; rocks; “Realistic Water”; track/ballast and
finally a hobo ‘jungle’ under an arch and two fishermen
trying their luck.
Making your own castings is not a major task, as this was a
different and fun project and certainly rewarding.
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