Alginate for Casting & Moulding, Chromatic Alginate Gel, Dental Impression Material... Plaster-of-Paris Life-casts..Plaster Cornices, Dados, Belly Moulds...

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Casting, Moulding & Modelling - Materials & Kits

All About Alginate

Alginate (pronounced al'jinate) is a flexible mould- and impression-making material; a gel is formed by mixing the alginate powder with water; it is sometimes referred to as 'chromatic alginate', because it changes colour according to the phase it has reached after mixing. The material is widely used in reproducing Decorative work, small to medium-size objects, figurines, etc. and patterns or mouldings in plaster and timber, particularly in restoration work. Alginate is completely safe in direct contact with skin (the same material is used for making dental impressions) and it sets very rapidly, so it is also the preferred material for Life-casting (making moulds of the face, body, feet and hands, etc.). Both types of work are explained in more detail lower down this page.

Alginate powder is usually supplied in a foil sachet of either a 1lb (450g) or 500g weight; it is manufactured from kelp - a variety of seaweed. In common with latex and silicone, it reproduces an extremely accurate impression - capturing such detail as the pores of the skin. Alginate has several advantages over silicone and latex:-

i) It sets very quickly and can be separated from the subject within 3 to 4 minutes of application.

ii) It is clean, economic, easy to apply and needs no special skill or tools, additives, etc.

iii) No release agent is required against any surface, the mould detaches from the original as it sets - although vaseline should be applied to avoid trapping strong hair growth, cloth fibres, etc., when used in life-casting.

Its drawbacks are:-

i) It is quite a floppy material once set and large, unsupported areas tend to sag and sometimes tear*; this often dictates the method used to make the mould (see below). *Serious tears can be repaired with supaglue.

ii) Alginate moulds are not particularly durable, and need using within 24 - 48 hours unless kept in humid conditions; however, using the mould soon after forming doesn't normally present any problems.

Methods

There are two ways of forming an alginate mould:-

i) Immersion. This is where the gel is poured into a suitable container and the object is immersed while the gel sets; the container can be a jug, a bowl, a sandwich box or even a plastic bag.

ii) Spread-application. A thicker mix is used than for the immersion technique, and the alginate is spread on to the object - usually by hand - and sets into a rubbery layer which can be peeled away after a few minutes.

Immersion uses more mixture - albeit slightly weaker - and is generally most-suited for small to medium-sized, free-standing objects. Moulds formed by spread application, however, can be very floppy and need support or a backing to preserve their shape - this usually has to be done before the mould is removed, but the choice of methods is usually fairly obvious. Depending on the shape and size of the original, either of these techniques may be used for making moulds of solid objects or in life-casting.

There are occasions when the topology of the original would make it impracticable to de-mould the alginate in one piece; however, by slicing through the set alginate (or even carefully tearing it where a sharp blade is not appropriate) - then matching up and strapping the sections together, it is possible to make moulds and produce castings from the most complicated originals; seam lines in the finishing castings can be easily be smoothed over later. An alternative is to mould and cast projections, extended limbs, etc., separately - which makes it easier to introduce reinforcement (dowels or wire) into slender areas or vulnerable points. Note that cuts and tears in alginate can also be repaired with supaglue.

Preparation and Technique

Note: The following general instructions apply to normal-setting (regular-set) alginate. The fast-setting alginate sets in a little less than half the times shown here. If using alginate for making moulds of small children's feet and hands, etc., similar setting times can be achieved by using warm or tepid water with normal-setting alginate - the warmer water is likely to be less disagreeable for the subject.

You will need to make all the usual preparations for casting work, including plenty of sheeting and newspaper, etc. on floors and work surfaces, a clean water supply and suitable mixing bowl and tools, etc, and some towels, and overalls or old clothes. According to which of the two methods you have selected you will need a suitable container or bag (to hold the gel for immersion-moulds) or suitable spreaders (the back of a table knife, plastic card, etc.) for spread-applications. If you are going to reinforce or provide a backing to a spread-mould in-situ, you will need to have the necessary materials made ready and cut to shape before you start. All the stage times quoted assume a water temperature of 20°C; using water 5°C lower may double these times, depending on air temperature, warmth of the subject, etc.

The powder to water ratio for immersion moulding is around 30:100 by weight; this roughly equates to one cup of loose powder to one cup of water - but, for consistent results, work by weight whenever possible. For spread-applications, a stiffer mix is needed - usually 40:100. A typical 450g packet of alginate yields around 1.3 litres of gel when mixed 40:100. A small trial beforehand is always a good idea using a kitchen scales for measuring out the powder, e.g. mix 40g of powder with 100ml water (100ml of water weighs 100g).

Mix the powder and water in a plastic or ceramic bowl which will hold about 4x the volume of water you are going to use - this will ensure plenty of room for mixing the paste. The alginate will peel off any smooth-surfaced container when it has set - and it's also easy to clean it off hands and tools then. Sprinkle the powder on to the water and stir in with the table knife using a firm, steady action taking care not to mix in air and small bubbles. Mixing will take around a minute in which time the paste will turn from purple to pink and all the flecks and lumps should have disappeared. Tap the sides of the container quite firmly before pouring to help release air bubbles.

Moulds for small objects can be made by pouring the paste into a polythene bag or a narrow container and then immersing the object/part, wriggling and moving it around for a few seconds to eliminate air, etc, then holding still for 2 - 3 minutes. Try to devise the container and arrange the subject so that there is at least a 6mm covering of gel at any point.

For larger surfaces, spread the paste evenly on to the subject with the back of a table knife, a suitable trowel, or a plastic card - or just use your bare hands - generally working upwards and outwards to give an all-over coating as quickly as possible, then immediately repeat to give an even and smooth finish. Avoid thinning/feathering out at the edges. Again, try to ensure that there is at least a 6mm covering of gel across the whole surface/profile - lesser thicknesses dry too quickly and will be difficult to remove, particularly on solid or rigid materials.

Finish work when the paste starts to turn white - the mix is then beginning to set and becomes grainy and difficult to use. The whole application should be completed within 60 - 90 seconds.

The set gel can be removed from the subject at a little under 4 minutes, using a gentle manipulative and soft pulling action to ensure gradual and even separation. Setting may take a few minutes longer on solid surfaces.

Note: On vertical and overhead surfaces the alginate will detach under its own weight towards the end of the setting period, so support must be given to the mould to prevent it falling away or tearing.

The mould will tend to dry out and deteriorate within a few hours, so start the casting fairly soon after forming - or keep it under a moist wrap (but not wet) with some appropriate extra support if the casting work has to be delayed. Depending on the particular product the alginate may turn back to pink or remain white after setting; dab off any excess moisture before (re-)use.

Decorative Mouldings and Ornaments.

One of the most common applications for alginate besides life-casting is the restoration and copying of decorative plaques, plaster mouldings and sections of cornices, etc. Forming the mould is much easier if the piece or section to be copied can be dismounted, and all the work done on a table or on the floor; however, this may not be practical - particularly when it comes to cornices and valuable, delicate or fragile fixtures that really can't be disturbed. Although working overhead is more awkward and likely to be more messy, producing the actual mould takes less than 4 minutes and is usually easier than it sounds. The freshly-mixed alginate is spread on by hand or with a suitable trowel (using a fairly thick mix), then a rigid strip or sheet of plastic, or plywood wrapped in plastic, etc. is pressed against it - effectively sandwiching the alginate. This backing* will ensure that the mould can be laid out flat afterwards so that the shape and lines of the impression are preserved. After tidying-up the edges with a sharp knife, a box can be assembled around the mould and the casting plaster poured in. For a section of cornice, for instance, the mould box will simply consist of two sides pressed against the alginate, sealed at the bottom, leaning in as if to meet at a right-angle - and just high enough to give a reasonable thickness of plaster across the alginate strip.

Reproducing a plaster cornice with alginate and plaster-of-Paris

Hessian, etc. can be laid into the wet plaster as reinforcement - this is particularly advisable when casting long sections of cornice, etc., but may not be necessary on small or deep sections, small plaques, etc. For large sections it may be feasible to cast just a thin, outer shell; conversely, for small, free-standing objects an immersion mould is often preferable - these alternative methods are described in ‘Life-casting’ below. Moulds that have little or no undercut can be often be used several times if a light baby oil is applied to the alginate between castings. *The backing strip or board often can be shaped or profiled to reduce the amount of alginate needed, particularly on long mouldings, etc. (alginate is surprisingly heavy).

Life-casting

The positioning and comfort of your subject is dealt with in the guidance sheets for different projects, as is work on face masks. After the alginate mould is removed you will probably need to give it some support, so towels, dish-cloths, etc. can be used to make a nest to prevent the mould falling out-of-shape or tearing, so have these prepared beforehand.

Skin and soft, downy hair will not require any release agent, but if the mould is likely to go over strong hair growth or clothing, etc. these will need to smoothed/smeared over with petroleum jelly.

Typically, a toddler's foot with an immersion mould in a plastic bag will use around 250g of powder (about 3 cupfuls of powder and 3 of water); A woman's hand or a man's hand (immersion), 600g and 750g respectively; a man's face (spread-application), 500g; a full mum-to-be belly-and-chest cast (spread-application), 1.5kg of powder. These amounts are approximate and are not particularly critical because the main objective is to ensure a thorough coating - it doesn't matter too much if a mould ends up at 10mm thick or just 5 - 6mm, provided good contact is made all over and there are no air pockets; work the alginate in between fingers, toes, skin wrinkles, etc., before the main application.

Immersion moulds: Moulds of children’s feet, hands, etc., or any small object, can be made by pouring the gel into a polythene bag, sandwich box, jug or bowl, etc., and then immersing the foot, etc., and wriggling and moving it around for a few seconds to eliminate air, then holding still for 2 - 3 minutes. Try to devise the container and arrange the subject so that there is at least a 6mm covering of gel at any point. Once set, ease the subject out of the mould as gently as possible using a back-and-forward action. Depending on the complexity of the mould this process may take several minutes.

Spread-application: For larger body areas, scoop up the alginate in your hands and spread the paste evenly on to the subject - working upwards and outwards to give an all-over coating as quickly as possible, then immediately repeat to give an even and smooth layer of 6 - 8mm. Avoid thinning/feathering out at the edges. Stop work when the paste turns white - the mix is then beginning to set and becomes ‘bitty’ and difficult to use. The whole application should be completed within 60 - 90 seconds. For small areas such as moulds of children's faces, etc., hessian or gauze can be dabbed into the unset alginate as the work proceeds and - provided it is well-supported after removal - this should give the mould sufficient strength for making the plaster cast; for larger areas, the set gel will need to be reinforced in-situ using modroc, but otherwise the mould can usually be separated from the subject at 4 minutes. A lot of spread-application work (as distinct from immersion-moulding) in life-casting involves other materials besides just alginate and plaster, and a number of kits are available on the main page that can be adapted for most situations (they include full instructions, etc.).

Very cold water and/or greater dilution will extend all the mix/set stage times. A small trial is always a good idea!

Plaster Casts from Alginate Moulds

If you expect to use the mould more than once, gently dab it dry and then wipe it over with a light baby oil before casting. Fine Casting plaster is suitable for static work such as decorative mouldings; for portable objects, life-casting work and items that are likely to be picked up or handled frequently, Herculite No.2 plaster should be used as it is much more damage-resistant, but if the final piece is going to be left ‘as-cast’, Crystacal R is preferable as it is a very pure white and is even stronger - but use a water:plaster powder ratio nearer 40:100 for solid work in Crystacal R.
Solid Casts: Pour the casting plaster in a little at a time, moving and turning the mould around as vigorously as its size and shape allows; firm tapping will also help to eliminate bubbles and air pockets - this is particularly important with small hand and foot moulds, etc. When the plaster has completely set and cooled, gently work the cast free from the mould. 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

Shell Casts: This is the usual method for making head, torso, and limb casts, etc. The cast is built up in several layers, starting off with an initial contact layer of plaster which is washed/bathed over the inside of the mould, then two or three layers incorporating hessian or fabric strips are gradually added (in much the same way as fibreglass work), with the finished shell being perhaps 10mm thick. Whole-body casts can be made by simply joining a back and a front cast together.