I am getting back to installing rock casting along a couple of steep slopes near the staging yards. I have been unhappy with using dilute washes of acrylic paint to stain the plaster rocks, as it is hard to control the colour you get, and any differences in plaster density show up as unnatural variations. This is especially noticeable when a different material is used to fill in between the plaster castings, such as spackling paste or some other filler. Patching between casting is necessary to blend the rock work into one seamless continuum, and any indication of where the joints are is very distracting. I had experimented with mixing special blends of different plasters to achieve the right density so that it would absorb the stain equally to the cast in place rocks, but this approach never really worked well and it was a lot of hassle.
A much better way was found during some searching online. I remembered that Joel Bragdon uses a plastic foam material for his “Geodesic Foam Scenery” approach. This hardens to a plastic that does not accept stain at all, so I figured that he must have a different way of colouring his rock castings. While I do not use his foam material, I thought the idea would be worth pursuing.
Joel has posted on his web site a very good article about how to cast foam rocks, install them, and colour them.
He says that because the foam will not accept stain, it is necessary to paint it with thin washes. The trick here is that he first primes the castings with artists “Gesso” before applying the washes.
He explains further why this is the best material for the job, because, basically, this is exactly what it was designed for – acting as a base to receive paint and other materials. I figured that this should also work for basic plaster rocks castings, and pretty much any other material, as the colour goes on top of the Gesso. Long story short, it works very well, and the bonus is that anything can be used as a filler for the castings, without having to worry about how porous it is (or is not!). And, if everything goes completely wrong with the colours it is very easy to simply apply another coat of Gesso overtop of everything and start afresh.
One small problem popped up with applying white Gesso to white plaster. It is not easy to see if it is all covered when you are looking at brilliant white on white. A simple solution is to spray a very light coat of brown acrylic paint wash on the castings to give them just enough colour to tell where the Gesso was. Any colour will do as it will not be seen. You only need enough to give a bit of contrast with the pure white Gesso. I tried tinting the Gesso but found that it messed with the wash colours too much. Best to have pure white under the washes.
Ok, so now I have a great way to colour the rocks, but what colours to use? Everyone says to use photographs of the rocks you are modelling, and that is a great idea, but how do you achieve a specific colour if you are not an artist? Well, the engineer in me said that I need some formulas to follow, so I made a sample casting and set about to colour it in a patchwork quilt of colour combinations so that I would know what worked well and what didn’t.
The casting was primed with Gesso in the usual way, and then stripes of colour were applied horizontally and allowed to completely dry. The next day I added the same set of colours going across the first set of stripes, so that I ended up with a checkerboard of colour combinations to use as a reference. Some of the combinations are completely useless to what I am modelling, but some other combinations look pretty good, and they are combinations that I would have never thought would work. This just goes to show how bad I am at judging colours!
An interesting observation is that for squares that have the same two colours applied, the end result can be quite different, depending on which colour was applied first. The last layer tends to puddle in the cracks and corners and imparts an extra measure to the overall look. Cracks tend to be in shadow and should therefore be darker, so it seems that the best approach is to always add the darkest colour last.
These two photos are a good example of just how difficult it is to capture specific colours reliably. Both of these photos were taken in the exact same place, under the same lights with the same iPhone, and yet the colour of the manila file folder beneath the casting is very different between them. All I can imagine is that the automatic white balance of the camera was affected by the different colours on the rock. The first is the more accurate colour, whereas the second has a very blue cast as witnessed by the grey stripe being almost purple on my monitor.
Now all I have to do is find the square that matches the overall colour that I want, and I’ll know the formula to achieve it. I can always add a third layer (or more) in spots to add subtle highlights, and this can be done at any time in the future, because it is just paint on the surface and not a stain that soaks into the plaster.
And, by thinning the acrylic paint on the brush by simply dipping it in water without a specific dilution will result in a natural variation of colour intensity, which is also a good thing.
Now I have about six feet of rocks to paint, so I better get busy!