Making an Artificial Rock, Part I by Dierk Neugebauer For quite some time now I've had a real interest in growing trees on rocks instead of in pots. I think this fascination must go back thirty somewhat years when I collected a rock from the edge of a lake. Confined to this rock were several trees and mosses, all told this growth was about 35 cm high. Eventually this rock ended up in the garden, where, over a period of years, one of the trees grew to "full size", as its root system was able to expand in the soil surrounding the rock. In the last few years I have continued to plant bonsai trees on flat rocks that I collected in my travels. I loved the look, but hated the weight! Moving the rocks proved to be a real back breaker! Thus my interest in creating a rock or rock-like surface that would act as the "pot" for one or more bonsai trees. There seem to be several options for dealing with the weight problem. One option is to make a rock out of a mixture of concrete, where a significant proportion of the mix is vermiculite or perlite or sawdust, thus reducing the overall weight. Some nurseries sell planters that are basically made in this way. Another option is to use fiberglass. This is the route I chose for the present project. Once again two options presented themselves: I could build a mold out of flyscreening and wood and directly apply fiberglass to its surface, or I could take an actual rock, make a fiberglass mold of it, and then use that mold to "cast" a lightweight fiberglass rock. Naturally I chose the latter, more challenging route. For the experiment, I chose a small, collected rock about half a square meter in size as my guinea pig. What follows in this article are the steps I went through in creating the fiberglass mold. The next article will deal with the casting of the rock using this mold. 1. First of all. I studied my rock, to decide which surface should represent the "top". Having made that decision, I then looked carefully for any areas of the rock that had "undercuts" in it. In other words, if the rock had any horizontal grooves or depressions, the fiberglass would fill those voids and then make it impossible to release the rock once the fiberglass had hardened. 2. I solved the undercut problem by filling these areas with a special glue that would stick to anything, and dry rock hard. The product is manufactured by "LePage", comes in cartridge form, and is called "PL Premium", ... and it works. It is a "polyurethane construction adhesive," and costs about 3-4 dollars a cartridge. Body filler would probably have worked just as well. 3. Although I had worked with fiberglass a number of times before, I had never attempted to make and use a mold. There were several questions that needed answering before I could proceed. Thus I visited a fiberglass "shop", where the owner manufactures fiberglass products for the world and for anyone interested in paying the price, and, most important, where the owner was interested in discussing my project with me and giving me invaluable advice. Finally, I was able to get all of my supplies from him as well, and I followed all of his suggestions. 4. Here is what I bought for about a hundred dollars: a gallon of resin 4 oz. of hardener 16 oz. of a liquid mold release a 1 lb tin of mold release wax a large coffee tin of fiberglass resin "paste" - it's liquid resin with "body": it contains fiberglass particles several 10 oz. containers of gellcoat colors that he had available: black, white, blue, and red; no yellow was available I already had leftover acetone, which is used for cleaning up, and several yards of fiberglass matting from earlier projects - these would probably add another $25 dollars to the cost. Do not buy fiberglass cloth (which has a weave, just like regular cloth does; matting looks like a felt and works better for this project.) the hour's worth of advice was free, and yet priceless! 5. The first problem I anticipated had to do with the rock's irregularly shaped bottom. How can I make a mold without "locking" the rock into the fiberglass as the mold is being made? My solution was to take some plywood that was larger by several inches all around, than the rock. I placed the rock on it, and traced its outline on the plywood. Then I cut the rock-shape out of the plywood in such a way that there was an extra half inch of space all around. Once the shape was cut out of the plywood, I centered the rock in the opening, and attached both plywood and rock to another sheet of plywood so that I had a stable base. Undercuts have been filled in the rock and it is ready for making the fiberglass mold. The plywood cutout has been made 1 cm larger than the rock, and the rock has been centered in it. 6. I next prepared the rock for fiberglassing by applying several coats of mold release wax, and a couple of coats of liquid mold release. Once the latter was dry, I was ready to begin. 7. I cut several pieces of fiberglass matting, ranging in size from a few square inches to about a square foot. Most pieces were longer than they were wide. the smaller the pieces, the easier to make them conform to an irregularly shaped rock! Matting should be cut as small as possible to help it to conform to the rock. Having all of your supplies laid out and available before you start fiberglassing is a must! 8. I attached a double layer of wax paper to a piece of plywood with some staples, in order to have a smooth base for applying fiberglass resin to the matting, before applying the saturated matting to the rock. Other tools that are needed are a stiff, disposable, 2 inch paint brush; plastic tubs; one for acetone, the other for mixing the fiberglass resin; several pairs of latex gloves; safety glasses; paper towels. An assistant would also be invaluable! Lay everything out so that things are readily accessible. 9. Work outdoors, or in a very well ventilated space! I mixed about a cup of resin and some hardener in a plastic container. How much hardener to add depends very much on the weather: work in shade, not sun; on a warm day less hardener is required than on a cold day; on a humid day more hardener is required than on a dry day. There are lots of variables, so be prepared to experiment. Ideally, you want the resin to remain workable for about 15-20 minutes. 10. Take a piece of fiberglass felt and place it on your wax paper. Use your brush to saturate the felt with fiberglass resin (into which hardener has been mixed). Lift the saturated felt and place it on the rock in such a way that you will eventually cover the whole rock with fiberglass. I started by applying the first pieces of felt both over the plywood and the rock, so that the fiberglass was tapered away from the rock - the idea here is to make sure that the fiberglass does NOT go underneath the rock anywhere, or the rock will not be releasable from the hardened mold! Use your brush in a stippling motion to help the felt to adhere to the rock - the idea here is to remove all air bubbles between the rock and the felt. Brushing the felt will quickly make a mess by picking up numerous fiberglass strands from the felt - so don't brush: use the brush to poke at the fiberglass in a stippling motion. 11. Unless you are planning on making numerous "rocks" from your mold, two layers of soaked, fiberglass felt should be adequate for strength. 12. When the fiberglass has hardened, remove the rock from the mold. I found that this was easier said than done! I used a rubber mallet without success to dislodge the rock. Finally I used a large screwdriver to apply a little leverage, and the rock popped out. What relief! Though not that easy to see, the resin-soaked fiberglass matting is applied to the rock and allowed to overlap onto the plywood, in order to adhere to it. The rough texture of the rock made it difficult to release the rock from the mold, but eventually, with some prodding, ...it came out. 13. I discovered that my mold had several problems in the area between the plywood framing and the rock itself, since the felt was basically hanging in the air. I filed some of the irregularities here off with a coarse steel file, and then used some of the "fiberglass resin paste" mixed with hardener, to smooth out any rough spots, and to make sure that there were no areas in the mold that were undercut, thereby locking in my artificial rock when the time came to release it. But more on that in the next installment.