Latex - Latex Liquid Moulding Rubber for Plaster-of-Paris Projects, Guidance for Moulding Materials....

MARAGON - ARTS & CRAFTS

Casting, Moulding & Modelling - Materials & Kits

All About Latex

The Material

Latex sets naturally in air (i.e. at room temperature and without additives) and is used to make flexible impressions (negatives) of originals/prototypes that have complicated or undercut detail, and/or where the mould itself will be used to produce casts made of an inflexible or rigid material such as plaster-of-Paris, concrete, clay, plastic and resin. Latex is easy to use, requires no special equipment or tools, is relatively inexpensive and is capable of reproducing extremely fine detail. Besides industrial and technical applications, latex rubber is often seen in model plaster- moulding kits.

Generally, the more porous the material from which the original is made, the more quickly the rubber skin can be built up because the original draws the moisture out of the latex. Plaster-of-Paris is ideal, fired clay is also very good; modelling clays will require a sealer unless completely dry; wax, wood, stone and concrete originals do not normally present problems, but impervious materials such as glass, ceramic and plastic originals all have much longer drying times. Some metals and alloys may react with the latex - the most obvious indication being significant discoloration with a weaker mould resulting, but this problem can normally be eliminated by using a varnish or shellac coat on the original. There can also sometimes be an unfavourable reaction with certain plasticines.

Application Methods

Depending on the size and complexity of the original, the latex mould may be built up by:-

i) Repeatedly dipping the original into the latex.

ii) Painting the latex on in several coats.

iii) Adding a thickening agent to the latex so that it can be spread on in a single coat.

This third method of making a mould is ideal for uncomplicated work or any large mould - typically those over 200mm high, etc. or over 300mm girth - and is described lower down the page.

Whichever method is adopted, drying shrinkage will be minimised if the freshly-made mould can be left on the original for as long as possible - preferably several days. The choice between dipping or painting is usually a matter of practicality, but dipping is quicker and more likely to achieve an even and consistent application, - without causing any disruption or disturbance to the preceding coats. Dip application does not usually require more than 3 immersions to build up the mould, but as many as 7 or 8 coats may be needed if the latex is applied by brush. In either case each coat is applied while the previous coat is still tacky, and the completed skin must be flexible enough but also strong enough to ensure that it will not be damaged when separated/peeled away from the original - and be sufficiently strong and flexible for the subsequent castings, bearing in mind weight, size, intricacy, etc. Depending on the size and material of the castings that it will eventually be used to make, consideration should be given at this stage to any special adaptations to the shape of the mould e.g. incorporating a rim or plinth in the pattern so that the mould can be more-easily suspended or supported to relieve the weight of the casting plaster, etc. Sometimes the shape of the original makes it impracticable to make a mould in one piece; however, by making the mould in sections it is possible to produce castings of the most complicated ornaments and figurines; seams or joint lines in the finishing work can easily be rubbed down later. Casting projections, extended limbs, etc., separately also makes it easier to introduce reinforcement (dowels or wire) into slender or vulnerable areas.

It is useful to know that if a latex mould proves too thin in use, it can usually be added to with fresh latex provided it is thoroughly washed in warm water and allowed to completely dry before applying the extra coats, etc. - preferably replacing the mould back over the original to do the work.

Preparation

Whichever method of application is used, paint brushes, tools, the latex container and dipping- pot must be kept sealed up and completely free of any possible contamination between successive coats. Realistically, to protect the work work from dust, the work should be carried out indoors with good ventilation, but away from draughts and open windows, and out of direct sunlight. Before starting work, take all the usual precautions with sheeting or newspaper on floor and work surfaces, with the right tools to hand, and wear overalls or old clothes. Utensils can be cleaned with water while the latex is still fresh.

Some means of holding the master during the application process is essential - a long wood screw in the base (or perhaps glued on if the original is very hard) is often the most effective method.

Dip Application

This method is particular suitable for porous originals having a shape or profile that lends itself to (safe) suspension during immersion. The dipping pot can be an old jam jar, a sandwich box or even a bucket - but it must be completely sealable between dippings because the latex dries very rapidly. Some method of hanging the object during and after dipping must be devised and the dipping process must ensure complete coverage without trapping air, etc; dripping should cease before the object is moved away from the dipping pot. Sometimes, because of excessive surface porosity, bubbles may develop in the initial coat, and these must be burst and re-dipped immediately. After about 15 minutes the object can be re-dipped; the latex should be touch-dry in 15 mins. at normal room temperature (20ºC), turning from white to a semi-transparent creamy yellow colour, and ready to peel from the original after 2 - 3 hours, but leave as long as possible to reduce shrinkage. Drying can be accelerated by heating up to about 70ºC - 75ºC. At least two coats are required. Apply talc or washing-up liquid to prevent the latex sticking to itself when peeling the mould away from the master.

Brush Application

To prevent the latex attaching to the bristles, wet the the brush before the first coat in fairly soapy water and squeeze out the excess; wash it out with warm, soapy water after each coat, and wet it again with soapy water before starting the next coat, squeezing out the excess before resuming. Apply a thick coating in each layer as you would ordinary paint, allowing 15 - 20 minutes before applying the next layer, i.e. while the previous coat is still slightly tacky, but is firm. As it dries the latex will turn from white to a creamy yellow or coffee colour. If the original is non-porous the latex will tend to run, so only very thin layers can be applied. Keep repeating the process until a skin of sufficient thickness has been built up - 7 or 8 layers may be necessary. If the original is pre-warmed in a very low oven, a thicker skin will form in the initial layer and this will often give better results. A hair-drier can also be used to speed up the drying process. Latex can be thickened by the addition of talc or very fine plaster but the amount should be judged from preliminary experiment, bearing in mind that there will be some loss of flexibility, and the initial coat should always be pure latex for the best detailing. Thixotropic additive can be used to make brush-on application quicker, using a mixing ratio of 2 to 3% without introducing the problems that are associated with full 'butter-on' application described below. Cotton gauze, surgical bandage or other fine fabric can be applied to the wet latex and within each coat as reinforcement to prevent the weight of the subsequent casts causing distortion.As for dipping, allow several hours (preferably much longer) before separation from the original, and apply talc or washing-up liquid to prevent sticking while unpeeling.

Butter-on Application

A thickening agent (thixotropic additive - or just 'thixo' for short) can be mixed into the latex. This converts the latex into a paste of about the same consistency as whipped cream; this paste can be buttered directly on to the original using the back of a table knife, etc. The original should be firmly positioned on a base-board and the latex pasted on, starting at the bottom and working up - with upward strokes of the knife, etc. - preferably applying a thin film all over first, so that visible bubbles can be eliminated - although this problem can largely be eliminated if an initial coat can be applied using un-thickened latex (either by dipping or painting). Because of the risk of inadvertently bridging over undercuts and fine detail, the butter-on method is best-suited for straightforward originals, but is ideal for most large projects. Inevitably, the outer surface of the newly-created mould will only be as good as the finish that can be given to it with the knife or trowel, etc., although this does not usually matter. Because thickened latex is often applied as one layer, full curing of the mould can take significantly longer. Thixo is normally added in the ratio of between 5 and 10%, very gently mixing in a small amount at a time until the desired consistency is reached. As a useful guide, a tablespoon of the thixo syrup is about 15ml and usually enough to thicken 150 - 200ml of latex.

Use and Care of Moulds

Allow the latex to cure properly for a day before using the mould for the first time, then wash it thoroughly in warm water with a mild detergent using a soft sponge or cloth and allow it to drain and dry thoroughly.

Larger moulds will need support to avoid any stretching and distortion from the weight of the wet casting material; tape or light strapping is sometimes sufficient, or the mould can be gently nestled into a box of dry plaster or sand etc., or suspended in water; moulds for deep/heavy objects may need a rigid jacket or casing of plaster-of-Paris, possibly made in two halves. Expanding foam contained between the mould and an outer box can be used to support surprisingly heavy castings, but note that a barrier such as petroleum jelly or plastic film is needed to prevent the uncured foam adhering to non-plastic surfaces, including the latex mould itself. If casting with resin, a PVA release agent should be applied to the mould before each casting and allowed to dry (it can be washed off with warm water).

If feasible, to reduce the weight and amount of casting material needed, a shaped piece of polystyrene or a plastic bag of sand, etc. can be positioned or suspended within the mould (while the plaster, etc. is being poured if necessary) to form a temporary core - so that the end result is a partially hollow casting.

When in use, depending on the material of the castings, the mould will need regular inspection and cleaning as above. When not in use, make sure the mould is completely clean and dry and is stored in a cool, dry situation, out of direct sunlight - and not positioned or poised at any angle that might cause distortion. Give the mould a light dustng of talc inside and out if you are unlikely to use it again for a while.