This is by no means intended to be a comprehensive guide to the finer details of all potential concrete casting surfaces. Instead, this article is intended to help you begin asking the right questions, as well as give you a basic understanding of some of the options. We cannot come up with solutions until we ask the right questions.
All the following materials should be tested with and without a mold release using your own mix design. Every different mix design, application technique and environmental reality can affect molds and release differently. There are variables that will change from region to region and from mix design to mix design.
Aluminum is lightweight and strong. It is not an inexpensive material and in certain situations can have a chemical reaction with concrete causing pitting and discoloration in the surface. This could be a cool effect except for the fact that it’s inconsistent in terms of whether you get it and what it looks like.
Clay can be sculpted for a number of purposes, whether used as an edge profile or for high-detail sculpting. It is easy to work with, and you can get tremendous definition with it if you are patient. If using it to create a basic round-over profile in your mold, an extruder to create “ropes” of clay is worth the investment.
Concrete is a material used often in large precast situations. Depending on the finish of the concrete, minor undercuts and texture in general have to be considered to ensure release. For large detailed applications it is an inexpensive and easy-to-produce casting surface. Finding the appropriate release agent is crucial — the choice of release directly correlates to the finish of the concrete casting deck.
Corian has found its best use ever as a surface used to cast nice countertops. Corian is a good option for reusable casting decks. It is easy to sand to whatever finish you desire, and it can be seamed into large decks. Seams and texture in the Corian will translate into the finished piece, but are easily polished out. Corian is a good option for sink molds and other rectilinear forms, and if you want to get brave you can heat and bend the material. There is a two-part glue that is the best option for fastening pieces together. Both double-sided tape and hot glue are ideal for attaching forms to Corian.
Dirt. Maybe you think this is a joke. But dirt can make some fine pieces. The consistency of the dirt plays a huge role in what the options are. Three common methods when using dirt are digging holes, creating impressions, and creating internal voids. You can dig a hole the shape of the concrete you want, then pour the concrete into the hole. You can also place an impression into dirt and cast the impression. Dirt is an inexpensive material for creating voids in large pieces that are to be hand-finished.
The most versatile soils are certain types of fine clays that allow good detail for impressions and are easily shaped when digging holes.
One consideration when using dirt is how the piece will be affected by the porosity of the soil. Moistening the soil prior to casting and applying oils and resins are ways to combat water loss.
Fabric is perhaps one of the most useful materials to gain recognition in recent times. Stretched fabric easily creates forms that are very difficult to achieve through other means.
Fabric as a form is used in two main ways, placed in the form and allowed to stretch under pressure, and prestretched and hardened. In both situations there is some type of armature that restricts the movement of the fabric, coupled with open space that allows room for the fabric to stretch. Fabric can also be used as an inlay purely for texture. There are a number of different fabrics that can be used, all of which have different stretching habits and different textures. This is a vast area that deserves an entire article.
Fiberglass relates to two main materials of pertinence, fiberglass molds and fiberglass resin. Fiberglass molds are some of the more durable and easily repairable molds available, particularly for multiple uses. Fiberglass resin is good for use in coating forms, whether they be made of wood-based materials or fabrics.
Be aware that the chemical reaction of fiberglass resin during curing creates a generous amount of heat in certain situations. In plain English, you can potentially catch your shop on fire. Resin should be laid in multiple thin applications to avoid catching things on fire. Fiberglass resin found at the big box stores has a considerably thicker consistency than what you order from a resin supplier. This can be a good thing or not, depending on what you are after.
Foam is a general term that can be used to describe a number of materials. The two that seem most worthy of discussion are high-density foam and sheet foam. High-density foam is good for using in CNC applications and in situations for carving. You can get different densities of foam and cast them yourself. Sheet foam can be used for a multitude of purposes. Its main benefits are that it is easily shaped, very available, and easily removed from molds.
Glass is mostly useful because of its sheen. If you cast on glass and cure properly (or get a mix overloaded with polymers) you will have a glasslike finish out of the mold. There are lots of types of plastics that achieve that same function with a bit less headache. Glass has plenty of obvious drawbacks in terms of durability and handling.
HDO, or high-density overlay, is used widely in the precast industry for forms. It is a plywood with a durable coating applied to it. It is good for many different applications, particularly for multiple uses. The pattern of the wood can show through the coating on cement finishes. This is particularly true after multiple castings.
Laminate is one of the more versatile materials available. It comes in dimensions up to 12 feet by 5 feet, it comes with and without texture, it is very reusable, it is very flexible and can take tight curves, it is very thin, and its uses are many. Our first reusable casting decks were made from laminate, and they held up beyond expectation. We use 12-foot lengths to mask seams in edges that are longer than 8 feet. We use it for curved forms, we use it for seams in molds, and the list goes on.
MDF and particleboard are useful sheet materials that are often more flat than plywood, and unlike melamine, they are ready for a coating. They are great for forms that will be coated with fiberglass resin or have laminate applied to the surface. Anytime these materials are being used, much attention needs to be given to ensure that water will never find its way into the material, unless you want it to swell up.
Melamine is one of the most useful casting surfaces available to us, albeit one that is plenty overused. There are different coatings used to create melamine, some more durable than others. It is not a great material for multiple uses, but it can be done. Previously used melamine can be used for edging and to build shop shelves, and is also great as a speedy snow sled. Perhaps there will be a contest someday to find the most creative reuses for melamine.
Paper is a great way to introduce texture. You can tear it, fold it or wrinkle it and produce some interesting effects. Concrete will bond tenaciously to many different styles of paper, so it needs to be well-sealed before casting.
Plaster is an Old World material of choice. The applications of this material are only limited by your imagination. Plaster can be jigged wet, cast and milled. Plaster can serve as a mold for direct casting or as a master to build other molds from. Care needs to be taken to seal the plaster before using it in a mold, regardless of whether you are using it as a direct mold or a master. There are lots of types of plasters to choose from, all carrying different properties.
Plastic sheeting and plastic films offer lots of options, whether a thin film or 1/2-inch thick sheet. Thicker plastics can be used for a reusable casting surface — you can build sink molds with it, and it can be used for reusable edging. Films can be used as a cover for a casting surface, but lots of care has to be taken to ensure that particles don’t find their way under the plastic, as this will cause divots that are very difficult to deal with.
There are lots of different types of plastics that can be used. Some work well with concrete and some do not. Before making a big purchase of any plastic, it would be recommended to test a bit to make sure that it is not affected by the chemical reactions or temperatures associated with newly placed concrete.
Plywood is limited in its function for molds, though it works well for reinforcing molds and as a textural surface for casting against. There are less expensive and flatter options for surfaces that will need to be coated with another material prior to casting.
Rubber offers plenty of uses, as well as plenty of variations. There are as many styles of rubber as there are uses for the material. There are a few companies that offer rubber, and I have found their technical staffpeople to be very helpful in choosing the right rubber for any given application. Follow the prescribed directions for mixing and using rubber, or you can experience some expensive failures. Because of rubber’s potential for flexibility, there are certain forms that can only be accomplished using rubber.
Tile board is an inexpensive shiny surface that seems like a wonderful option as a casting surface. We used it in the early days with both successes and failures. The inconsistencies in manufacturing eventually became so difficult to manage that we stopped using it altogether. The coating on tile board is not very durable, and one little scratch that is only visible in certain light can cause an undesirable result. On the flipside, you can intentionally scar it up and achieve some very interesting effects. On one of our first pieces we used a stick vibrator directly in the form, which forced the aggregate to bounce against the tile board, creating a very interesting pattern.
Steel casting tables might have lost favor recently, but they seemed to be the reusable casting surface of choice a few years back. Steel is a very durable material that can last forever. It wouldn’t be the material of choice if you are after a cement finish, but pieces that will be ground will come out fine. I have heard horror stories of the wrong release being used with steel and having the concrete bond to the table. If you decide to go this route, seek advice from someone who has experience with this surface.
Stone slabs, as with Corian, found their highest calling when they started being used as a casting surface for concrete. If you can find the right stone, this can be a casting surface that is dead flat, lasts forever, and requires very little maintenance. Picking the right stone, with minimal patterning and variation, is key to finding success down this road.
Wax is a useful material for details in concrete. Hot wax can be cast for details like drain grooves and the like, keeping in mind that many waxes shrink as they cool. Wax can also be carved and torched for nice sculpted details. Some waxes can leave residue on the surface that has to be tended to after casting.