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General
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The simplest way of making rocks on a model railroad is to copy mother nature. The process to do this
is even simple too. Latex molds are made from real rocks and are then used to make plaster copies for
mountains, rock outcroppings, etc. Once a scenery base is completed, the latex rock molds are filled
with plaster and applied to the scenery base. After colouring with stains or paints, the plaster rock
will look like the real thing.
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Real Rock
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Select rocks from nature that have surfaces similar to the rock effects you want on your scenery.
For jagged type rock faces, a rock called "Feldspar" is especially good. Jagged type rock can also
be found at construction sites and road cuts where blasting has taken place. This yields a rock
face with sharp edges because nature hasn't had time yet to weather wear the features. It is
important to select the rock based on fidelity of detail when viewed in scale dimensions, the finer
the detail, the better. Once you have the rocks, you must clean them. This is where your neighbours
will be thoroughly convinced that you are certifiably insane when they see you out in the driveway
washing rocks!
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Latex Mold Material
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Lewiscraft & Micheals sell a products like LiquidTex which are latex based mold materials used to
make Halloween masks, etc. The LiquidTex is painted directly on the cleaned rock face. A quick
spray of water on the rock surface will ensure the latex material gets into every nook and cranny,
The latex mixes with the water and will flow into every rock feature. After each coat dries, simply
paint on another layer of LiquidTex. The number of coats depends on the mold stiffness you want.
Some like an extremely flexible mold - this would require 6 or 7 coats. A flexible mold is good for
wrapping around scenery features but, there's a down side - the mold will wear quicker and holes will
begin to develop. However, even if a hole does develop from wear, simply touch up the hole with some
LiquidTex and you're back in business. A stiffer mold will need at least 8 coats and then some
supporting material such as pantyhose or fiberglass screen which is painted in with a LiquidTex coat.
Another coat or two of LiquidTex and your mold is finished. One down side to a stiffer mold - you won't
be able to wrap the mold around scenery features thereby restricting the mold applications.
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Mold Plaster
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The recommended plaster for rock molds is Hydrocal though Plaster of Paris will also yield acceptable
results. Mix the plaster as you normally would and fill the molds that are laying on a flat surface.
The approximate mix ratio I use is 1 part water, 2 parts plaster. If needed, prop up sections of the
mold so the plaster doesn't run out of the mold.
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When To Apply Molds
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There are two approaches: dry molds & wet molds.
Dry Molds - The dry mold process basically involves waiting until the plaster has set in the mold.
The plaster rock is then removed and glued to the scenery. The down sides to this process are; the
rocks faces are flat, and there invariably gaps left around the mold. To resolve the latter, plaster
is applied around the mold to blend the rock in with the rest of the scenery and surrounding castings.
The up side is that timing is not an issue. Just wait until the mold sets and then glue the rock in place.
Wet Molds – This is the more popular and versatile method. Mix the plaster and leave it in the mold
for approximately 2 minutes. This time will vary depending on the plaster/water ratio, humidity and room
temperature. A 50/50 mixture of plaster to water will take about 4 minutes. A mixture of 2/3 plaster to 1/3
water will only take about 2 to 3 minutes to set up. The 50/50 mixture will allow more working time to blend
in the wet plaster to the scenery base and existing rock molds. To determine when is the “right” time to
apply the mold, I look for when the plaster has started to loose its “wet” sheen and forms slight cracks
when flexed. If the plaster is left in the mold too long before applying, the plaster will form hard cracks
(break) and will not bend to the contour of the hard shell. If this happens, it is too late to apply the mold.
Chalk this up to experience and start again.
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Applying and Blending Molds
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Dry Molds - The dried plaster rock is simply glued on to the scenery base with white glue, panel
adhesive or more plaster. Plaster or other scenery filler type material, such as Sculptamold, is used to
fill in the gaps left around the plaster rock and used to help blend the rock to the surrounding surfaces.
Wet Molds - Before pouring the plaster in the mold, you should have a good idea exactly where the
mold will be positioned. When the plaster is ready, begin setting the plaster to the base by lightly
pressing the mold against the scenery base starting at the centre of the mold. Work the plaster from the
centre towards the outside of the mold until it oozes out from the edges. The plaster that oozes from the
edges will be used to blend the molded rock to the surrounding surfaces. A butter knife is a good tool to
blend the mold to the surrounding surfaces. Use whatever works best for you. Basically, carve the oozed
plaster to existing shapes next to the molded rock. If the rock is next to flat surfaces, work the oozed
plaster from the mold edge gradually into the flat surface. The plaster rock should appear to emerge from
the flat surface. If the molded rock is next to another rock, use the knife to carve the oozed plaster
similar to the rock face it touches. You can also let the oozed plaster set for 5 – 10 minutes, then flick
it off with a sharp instrument. The plaster rock should appear to be one continuous rock face with the
other rock molds.
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Bob demonstrates the Wet Method with the application of a rubber mold with plaster.
The rubber mold will be peeled off after app. 10 minutes to review an exact copy of the
mold's detail. |
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Plaster Rock Stains
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Water based paints and acrylics are used as stains to colour the plaster rocks. Using stains work best
because tinting and hues can be altered easily during the colouring process. There are 3 acrylics that
should be used because they best represent earth tones - raw sienna, burnt sienna and raw umber.
Traditionaly Universal Tinting Dyes have been used for colour which come in the hand full of colours.
These come in small tubes and are available from most paint stores.
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A more recent trend has been to use Acrylic craft paints that come in small squeeze bottle in a plethoria
of colours. In either case, you will need to dilute the colours with water to make a stain.
The rest of the colouring depends on whether you want to use gray tones or brown tones for your rocks.
Pick a base colour - for example, gray. Mix a large batch with water. Using another container, mix some
of this batch with some white paint thereby lightening the gray colours. Using other containers, use
colours to add different tones and even black to darken the mix.
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Colouring Process
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From experience, I have found that plaster that is not thoroughly dry will take the stains differently
than its neighbours. Therefore, I leave my plaster for about a week to ensure all portions are thoroughly
dry and have the same moisture content. Using the stains, begin by dabbing hap-hazardly the lighter colours.
Then start dabbing the darker stains inter-mixing from each of the different containers. There is no
sequence to this. The colours will start to flow into each other resulting in a colour combination you want.
Use a spray bottle with water to blend the colours together. A quick stroke with a brush may also be
required. If it doesn't happen the way you want, keep dabbing colours. In the worst case, let everything
dry and start again. You can start again and keep trying until you get the colour combinations you want.
This must be experienced - it is not a magical formula. One trick to point out... keep using the water
spray to remove or, alter the colours you have applied. If all else fails, wait until it dries and start
again. |
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Creating Rock Shadows
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This step can be performed before or after the castings have been coloured. I choose the later course
as it allows me to “tone down” the brightness of the colours. A mixture of India Ink and water is
sprayed onto the completed castings. A spray is necessary to ensure the black ink gets into all the
nooks and crannies as this is where the shadows appear. Allow the spray to puddle and run down the surface
of the rocks as this will help accentuate the cracks, etc. |
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End Result
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This whole exercise is one of doing and refining as it is not an exact science. Feel free to experiment.
Until you try plaster rocks and colouring them, you can never understand how they are done.
Try it...you'll like it...The worst case scenario will only result in lost time.
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