a) Rabbit-skin Glue granules
b) Double boiler ( or two pans that fit inside each other)
c) 2-3" (5-7cms) brush
Canvas (or linen) is sized to reduce the absorbancy of the surface and to prevent the fibres from absorbing any oil either from the ground or from the oil paint, which otherwise, over time would rot the fibres of the material.
But the size itself has to be as flexible as possible to cope with the movement of the canvas and should not be a thick heavy layer, which would inevitably cause cracking in the subsequent paint layer.
The best and most suitable size available is a natural product - rabbit skin glue - made from animal skin and bones. Being a natural product, only approximate measurements can be given, and a degree of trial and error is required. But as a rough guide, anything between 1 ? to 2 ? ozs (30 - 45g) to 2 pints (1.137 litres) of water will be about right.
The only sure way of knowing that you have the right strength is to prepare a batch and test the set of the cold size. Rabbit skin or animal glue is hygroscopic, and if too strong it will absorb too much moisture from the air and the paint layer, taken to extremes, can peel off. If too weak it will offer no protection and the ground or oil paint will sink into the cloth.
As a starting point put 2oz (40g) of glue with 2 pints (1.137 litres) of water into the top part of a double boiler.
For this initial batch of a new bag of glue granules, it is easier to estimate the proportion of granules on the high side as it is easier to add water to weaken the mixture than to re-soak granules to add to it.
Leave the granules to swell in the water for about 2 - 3 hours. Once the granules are uniformly swollen they are ready to be heated in the double boiler, stirring occasionally untilfully melted to a uniform grey-beige colour. Set the glue to one side and allow to cool fully, which depending on the weather conditions can be anything from between 2 to 10 hours.
The way of testing the strength of the size has not changed in hundreds of years, and there is really no substitute for it. Using your forefinger and thumb, split the surface of the size, so that the jelly is broken apart.
The sides of the walls of the crack should be uneven and rough, not smooth, and if worked with the thumb and finger should have the consistency of apple sauce. If more like jelly it is too hard and will need more water, any runnier it is too weak and will require more granules (which have to be soaked before they can be used.). In either event the size has to be re-heated before the amendments can be made.
To use the size it needs to be re-heated before its application to the linen or canvas, the heat ensuring even distribution. It will be hot enough if the size feels hot when your finger is put in it. On no account should the mixture be boiled as this will reduce the glues' effectiveness.
It is best to apply the glue size using a 2 -3" brush, avoiding overloading it, so as not to flood the surface with size. In applying the size you should brush it on in short strokes, avoiding back and forth movements, and avoiding going over previously covered areas. This will result in too much size being applied, and will be as if the size was too strong. Size the sides of the canvas as well as the surface, for even tension. Only one coat of glue size is necessary.
Leave the canvas to dry flat, and naturally, and do not use artificial means of drying, such as a hair dryer. The newly sized canvas will take at least 12 hours to dry in normal conditions.
Sizing will cause the canvas material to stretch slightly, so to ensure a taut working surface, it is best to temporarily stretch your canvas and re-stretch it after sizing. Linen in contrast will tighten with sizing so this precaution will not be necessary.
A new batch of size will keep for a week in the fridge, but be aware that too many re-heatings will gradually weaken the size.
email me with any questions and I will endeavour to sort out any problems you may have.
Skip to Top of Page