Making Plaster-of-Paris Moulds Although it can be sculpted as a 'negative', more usually a plaster mould is formed from an original or prototype. This may be part or all of an existing object, or a master modelled for making one or more copies, and might be a pot, a plate, a figurine or even part of a car. Originals made of pliable materials such as plastic, rubber, silicone and plasticine do not normally need a release agent (e.g. vaseline), but heavily-detailed, rigid or shiny surfaces can be difficult to part - metal, glass and ceramics all require a release agent, but see 'Flexible Moulding' below for the materials and methods used for copying relief and undercut detail. New and existing plaster readily adhere, so a release agent is needed if the mould and the original are both plaster-of-Paris, and the same applies if the subsequent casting is also plaster. Unless the original is itself flexible, any recesses and udercut areas will need temporary filling. Intricate or hollow moulds can be formed in sections which can be held together with rubber bands or tape, etc., and the joint-lines levelled off after separation. ¶The mould box can be any simple plastic container or frame lined with a plastic sheet, etc., with the object being firmly fixed or suspended in the box - tupperware containers, washing-up bowls, old cardboard boxes, even plastic footballs cut-in-half are all perfect. Plaster expands slightly as it sets, so straight-sided mould boxes that cannot be dismantled need to be built very slightly wider at the open end. If containment is difficult - as with a large panel, or other fixture - the mould box will have to be devised around or over the original ensuring that the junction with the sides of the box is completely sealed with plasticine, etc. Hessian or wire mesh can be included to add strength to slender or large areas of the mould. If the original is made of a soft substance such as paraffin wax which can be scooped or melted out of the mould, whole-object castings can be made (vents can be drilled into the mould to prevent air being trapped). Fine Casting or Artists' plaster is suitable for most work, but Herculite No.2 plaster can be used for moulds that will be used repeatedly; for maximum durability, however, use Crystacast. Setting time depends on the plaster used, but is usually under 30 minutes. ¶Semi-flexible casings or moulds can be made with Plaster-of-Paris bandage using the same material and technique as plaster-casts for bone fractures (see Body Moulding below). Making A Cast When it comes to making a cast from rigid or flexible moulds, Fine Casting or Artists' plaster is usually quite sufficient if the finished work is unlikely to be handled much. A typical plaster for more portable objects is Herculite No.2, but Crystacal R should be used if true whiteness is important. For maximum hardness use Crystacast or, for the best resistance to breakage, use Cassini's. (Cassini's can also be used for outdoor work and water features). Moulds must be completely dry if used for casting metals because the steam from trapped moisture can cause molten metal to be blown out - the mould may need drying in a low oven before use (not a microwave!). Any under-cut detail in the mould will prevent separation if the casting is plaster, metal, etc. - but this won't matter if the casting is a one-off and the mould can be broken away without damaging the finished article.
Flexible Moulding
Alginate, Latex Silicone, Vinyl Rubber (Vinamold)
A flexible mould is normally used for originals with any relief or undercut detail, etc., and there are several ways this can be done. 1. Alginate
can be used for making life-casts as described lower down but is also widely-used for copying figurines, small ornaments and decorative mouldings - especially in restoration work; alginate comes as a powder which is mixed with water and can either be applied as a paste or, using a weaker mix, the original can be immersed in the gel; the mixture sets in about 3 minutes and the mould detaches itself in the process. Alginate moulds are only suited to a very limited number of uses as the material degrades quite rapidly. 2. Latex moulds are often seen in children's casting kits but are also used extensively in the production of objects ranging from chess-pieces to heavy garden ornaments; the raw material is a milky fluid and moulds can either made by repeatedly dipping the original, by painting the liquid on in several coats or - by adding a thixotropic agent (thickener) - the latex can be buttered on in a single coat. Unthickened latex is ideal for moulds that have a distinct girth (as opposed to flat or linear profiles), but thickened latex can be used in any situation where there is not too much heavy detail.
3. RTV silicone moulds can be formed by placing the original in a pot or container with the liquid rubber being poured in the same way as a plaster block-mould. (RTV silicone is a two-part material and is very different to the silicone used in mastic sealants). A more efficient, two-stage process is often used for larger moulds*: a layer of clay or plasticine is spread over the original; this is then placed in a suitable mould-box and plaster-of-Paris poured in; once set, the plaster case is lifted off and the clay filling removed; when the plaster case is put back over the original there is a void where the clay was; the silicone to form the mould can now be poured into this cavity through holes formed in the plaster case - then left to cure as normal. The plaster case itself may also be used as a mould support for heavy castings. This technique can be adapted for making moulds of wall fixtures, plaques, etc. - effectively turning the assembly on its side. Silicone combines durability with great accuracy - it is therefore widely-used for small to medium-size ornaments and in engineering work where planes and angles need precise replication, and also for copying inscriptions, etc. As with latex, with the same limitation, a thickener ('thixo') can be added to the silicone so that it can simply be buttered on to the original to form a mould. 4. Vinyl rubber (Vinamold, etc.) is similar to silicone but the material is first heated in a pan to make it pourable - small projects can easily be managed in the microwave; for best results, the original should be pre-warmed so that the molten rubber can fill all the detail before solidifying; vinyl rubber can only be used on heat-resistant originals and is not as durable as silicone; however, moulds can be melted down and the rubber re-used. ¶Mould-boxes are rarely very elaborate - plastic food cartons and cut-down milk bottles are ideal. Release agents are not normally needed for alginate or latex, either when making the mould or when making castings; both silicone and vinyl rubber can also usually be persuaded to de-mould from most of the surface types that they are likely to be used with. Vaseline, WD40 and most household polishes, baby oils, etc., can be used to make separation easier, but shellac can also be used to seal particularly difficult or porous originals beforehand. ¶When in use, larger moulds may need support to avoid distortion from the weight of the material used in the casting; tape or light strapping may be sufficient, or the mould can be nestled into a box of dry plaster or sand or, if the mould is formed with a lip, it can be suspended in a jug of water; a mould for a deep/heavy object may need a rigid jacket or casing of plaster-of-Paris, possibly made in two halves strapped together. ¶The choice of plaster for casting from flexible moulds is much the same as for casting from rigid moulds (see above). ¶Casting resin reinforced with cotton gauze is also a very adaptable mould-forming medium and easier to work with than fibreglass; resin is also a popular medium for ornaments made from flexible moulds and can be mixed with marble powder, etc. and various decorative effects can be achieved with pigments and metal powders. Fastcast resin can be used for quicker results when making sets of models, chess pieces, etc. Clear-cast resin can be used for encapsulation, e.g. in paperweights. Use the tabs above this table to read more about alginate, latex, silicone, vinyl rubber and resin.
Sculpting
A plaster 'block' can be made in a basic mould that slightly exceeds the model's shape and size; the block may be made partly hollow by including a light infill foam to save on plaster, and weight. Sculpting is usually carried out when the plaster is quite dry otherwise tools and sandpaper, etc., will clog. Repairs are easily effected since new plaster readily adheres to existing. Crystacal R is ideal for sculpting as it is a very pure white. Cassini's can be used for outdoor work.
Built-Up/ Armature Work
A layer of light plaster (typically Fine Casting) is built up around a hollow cage or framework (armature). Large pieces such as life-size figures will require wire and rods (not necessarily metal) to make up the frame, to which wire mesh, etc., can be attached. Small pieces can be made using mesh alone. Canvas or cloth is then dipped in wet plaster* and then wrapped over the parts of this frame in successive layers until a sufficient covering and the right shape is achieved. (In the early stages any parts that protrude too much can usually be knocked back in - slightly too far - then filled out again). The final piece can be left natural, sculpted - or smoothed over with a final coat of plaster. *Modroc pre-coated plaster bandage is usually more convenient for small and medium-sized work.
Impressions/
Imprints
Imprints are made by pressing the subject into the wet plaster, so that the mould itself is the finished item; a release agent such as a vaseline may be needed depending on the material (see Casting below); this technique is popular with parents who want a record of the size and shape of small hands and feet, and a kilo of plaster and two plastic flower-pot bowls is all that is needed for several small hand and foot imprints. Children's Hand & Foot Imprint Plaster Pack
Slip-Casting
Liquid clay (slip) is poured into a plaster-of-Paris mould. The plaster absorbs water from the clay to leave a hard layer; excess slip is poured away. As the clay dries, it shrinks slightly, detaching itself from the mould surface; once the clay is firm enough it can be taken out of the mould - which is then dried for re-use. The process can be adapted for making pots, etc., using split-moulds (the slip is bathed over the inside of the mould and the excess drained off) - or for solid objects, including tiles, but is not suited to heavy detail. Denser plasters can be used for high-quality work, however lighter plasters absorb water faster, allowing a shorter mould cycle.
Body Moulding
This single-stage technique is very popular with mothers-to-be who want a memento of their body shape during pregnancy. A pre-coated plaster-of-Paris bandage* is used to make the mould. Tummy and chest are first covered with petroleum jelly, then two or three layers of the moistened bandage are applied more-or-less as a thin poultice which sets very rapidly. The whole process takes 15 minutes-or-so and, although the result is a solid veil, it captures all the shape and holds a surprising amount of detail - and is great fun to decorate. Nursery scenes, cartoon and animal motifs are very popular, but in the right lighting and position, a plain, natural look can be very dramatic.
View the Body mould & mum-to-be kit. There is more information about this simple moulding technique via the link above this table. *Modroc bandage is a pre-coated plaster bandage which is dipped in water for 3 or 4 seconds - it then becomes quite 'pasty' and is applied/draped over the skin and then smoothed into the body's contours.
Open gallery.
Life-Casting (Face, Body, Hands, Feet)
This involves making a flexible mould which is peeled away from the subject once set; the mould is formed in alginate gel which sets in 2 - 3 minutes and will even hold such detail as the pores of the skin. This is the negative which is then coated or filled on the inside with plaster-of-Paris; when the mould is removed the end-result is an exact plaster replica. The method is popular with parents who want to make a cast of their baby's foot or hand. The foot is either dipped or 'painted' with the alginate which can be peeled off once set (alginate is completely safe against the skin). The resulting 'sock' is firm and strong enough to then be filled with casting plaster. A hard plaster such as Herculite No.2 is normally used for life-casting, but if the piece is to be left 'as-cast' a pure-white plaster such as Crystacal R can be used. The cast of the 'Hand Emerging' shown below was made in a tuppperware box and was completed in just 30 minutes.
Working with Alginate - Casting a Hand
On larger body-areas the alginate will need a supportive backing: if the alginate is applied directly to the tummy and chest and then bandaged in-situ as for the Body-Mould process above, the result is a mould which is sufficiently flexible that it can be separated from the body without distortion; plaster and layers of fabric are then applied on the inside to create the plaster replica, which is a shell rather than a solid mass. This is the method used by mums-to-be who want a naked-skin replica of themselves at the later stages of pregnancy, and is definitely a two or even three-person project, requiring 20 - 30 minutes of close co-operation to make the mould. The casting is often painted - metallic spray is very impressive. Life-cast & mum-to-be kit. Using the same technique, whole body or whole head casts, etc., can be made by casting front and back separately then joining them.
General casting and moulding; suitable for a wide range of artistic applications and most large objects and impression work; also fibrous plasterwork, decorative mouldings, panels, etc.
Decorative items, occasional moulds, impressions, models, etc. Large items may need an armature. Artists' is slightly stronger than Fine Casting plaster.
Sculptures and casts requiring durability, reusable moulds and items likely to be handled frequently. Longer drying time in slip moulds but greater detail. Suitable for life-casting if painted, etc.
Dense hard surface; large casts and projections may require the inclusion of mesh or fibre, etc.
Displayware, giftware, chess pieces, etc., reusable moulds; slower drying in slip moulds and slightly better definition than Herculite No.2, and much-favoured for life-casting work.
Dense hard surface; very popular in exhibits, displayware and undecorated life-casting because of its whiteness.
Statues, figurines, water features, sculptures, impressions, etc. Also marble/stone repair & restoration (cast or applied). Suitable for both indoor and outdoor work.
Water/weather-resistant; dense surface; very good impression capability and the best resistance to breakage and chipping.
Moulds or casts requiring extreme durability, or where the finest possible detail is necessary, e.g in ceramics.
Extremely dense surface; Crystacast is one of the finest pottery plasters available; Herculite Stone is very chip-resistant.
Delivery is usually the next working day for most of the UK, but please allow an extra day for 'Highland and Island' addresses and larger orders. Sepal-Maragon, 1 Woodhall Farm, Hatfield, Herts, AL9 5NU. 0800 6347410 & 01707 276156 enquiries@maragon.co.uk. VAT Reg. 911 1205 86.
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