Complete Artist’s Manual: The Definitive Guide to Materials and Techniques for Painting and Drawing. Simon Jennings
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Preparing size
The ingredients should be carefully measured to produce the required strength (see here). If the size is too strong, it forms a brittle layer which could cause the primer and the painting to peel and crack; too dilute a size will produce a weak film which allows oil from the upper layers to sink into the canvas.
Place the dry granules into the top part of a double boiler. Add the water and leave for about two hours to swell. Heat the resulting solution gently in the double boiler until it has melted, stirring until all the granules have completely dissolved, and never allowing the size to boil – this will destroy much of its sizing qualities. If you don’t have the use of a double boiler, you can heat the glue in a bowl standing in a pan of water (as shown in the illustration, right).
Set aside the glue for a couple of hours, to cool and form a jelly. Keep the container covered to prevent any loss of water through evaporation, and to protect from dust and flies. Test the strength of the glue with a finger – the surface should be rubbery, yet just soft enough to split. The split formed should be irregular; if it is smooth and clean, the size is too strong. If this is the case, just rewarm it, add water, and allow it to reset. If the size has not set, you can stir in up to 10g (¼oz) of glue and then leave to soak for 12 hours.
Preparing size in advance
If you are mixing up a batch for later use, it is useful to note that glue size can be kept in a refrigerator for up to a week before starting to decompose.
Applying size
Rabbit-skin glue is a strong adhesive and must be used thinly, or it will crack. One thin coat is sufficient to size a canvas; too thick a layer forms a continuous, level film on the surface, and prevents the subsequent priming layer from bonding with the canvas. Gently reheat the size until it is just lukewarm and almost jelly-like in consistency. Apply it to the canvas in a thin layer, working quickly before the size begins to dry. Start from the edge, and brush in one direction only – do not make a back-and-forth motion with the brush, as too much size will be applied. Size the back flaps and edges of the support as well as the front. Leave to dry in a dust-free place for about 12 hours before applying primer.
Temperature
Size may be applied hot to panels and boards, but on canvas it must be applied lukewarm. If too hot, it will soak through and glue the canvas to the stretcher and you will need to prise it free with a palette knife. Hot glue size may also cause fabric to over-tighten.
Sizing boards and panels
For boards and panels, use the slightly stronger solution described on the opposite page. Thin boards should also be sized on the reverse and edges, to prevent warping. Leave to dry for 12 hours, then sand lightly.
Rabbit-skin glue
This is the time-honoured size for rendering canvas impervious. Available in granule form, it is dissolved in hot water.
Recipes for glue size
These measurements are a good starting guide, although you may wish to vary them slightly.
For sizing canvas:
You need 55g (2oz) – two rounded tablespoons – glue to 1.1 litres (2 pints) water. (Alternatively, use 1 part by volume of glue to 13 parts water.)
For rigid panels:
Use a stronger solution of 85g (3oz) glue to 1.1 litres (2 pints) water. This recipe will make enough size to cover a support measuring about 120 × 180cm (4 × 6ft).
Improvised double boiler
An effective substitute can be made from a bowl or clean tin can heated in a pan of water.
Sizing implements
Brush size in one direction only
The best brush for sizing is a flat hog varnishing brush, with a good width and long bristles. Decorators’ brushes can be used, but poor-quality ones may shed hairs. Some artists use a natural sponge, which gives more control; gently squeeze out more glue when you feel the surface going dry. It can also be used to mop up any excess.
Unprimed supports
The warm brown tone of surfaces such as hardboard, plywood, cardboard and linen canvas provides a middle tone which can be incorporated into the painting. To make them suitable for painting on, while maintaining their colour and texture, seal them with a coat of dilute glue size (for oils) or acrylic medium (for all media). Here, Ken Howard uses very thin, turpentine-diluted paint, so that the canvas colour shows through. This gives a marvellous impression of reflected light on the model’s back.
Ken Howard
Homage to Lautrec
Oil on unprimed canvas
40 × 30cm (16 × 12in)
Alternative glue size
A modern alternative to traditional glue size is carboxymethyl/cellulose (CMC). This expands and contracts at the same rate as the canvas, greatly reducing the risk of cracking. It is also easier to use: just dissolve the granules in either warm or cold water (using an 8 per cent solution by volume), leave to swell and apply with a stiff brush. There is no heating involved – and no smell.
SEE ALSO
| PRIMING | The primer, or ground, not only seals and protects the support, but also provides a base that will readily accept the application of paint. |
Choosing primer
There are various types of primer, each with its different characteristics. It is important to choose the right one for your needs, as it affects the way paint is ‘pulled’ from the brush, and its finished appearance. For example, if you like to work on a smooth surface, you will require a different ground to someone who prefers a slightly textured, dryish surface that gives the paint a matt, chalky appearance.
In addition, it is vital that you select the right type of primer for your chosen support. Canvas expands and contracts, and thus requires a flexible ground; therefore an inflexible gesso ground is not suitable.
The ground should be absorbent enough to provide a key for the paint, but not so absorbent that it sucks oil from it – a common cause of sinking (the appearance of dull patches of paint across the canvas).
Oil primer
The traditional, and best, primer for oil painting, particularly on stretched canvas, is oil-based lead-white primer. This is flexible,