Material & Techniques

Canvasses used as support for painting are categorised according to the type of material, weight, and texture. The type of materials used to make canvasses are natural, synthetic, and hybrid. The weight classifications are light-weight, medium-weight, and heavy-weight. The textures of the canvasses are grouped into no grain, extra-fine, fine, medium, rough, and extra-rough. Additionally, artists have the option of purchasing canvasses that are pre-primed and stretched.

The strength and durability of a canvas are determined by the type and quality of the fibre, thickness of the thread, and the tightness of the weave. A canvas used for painting must be able to maintain its structural integrity when subjected to changes in humidity and temperature and resist tear, impact crack, and puncture. A loosely woven canvas will not be able to securely hold the sizing or ground in its interstices when subjected to changes in the environment. That in turn increases the risks of cracking and delamination of the ground and paint layers over time.

The tightness of the weave corresponds to the number of threads per square inch or threads per 10 square cm. The weave runs in two directions. The length of the canvas is made up of the warp threads and the width is made up of the weft threads. Canvasses with loosely woven threads are said to have an “open” weave and those with closely woven threads are said to be tightly woven. A tightly woven canvas will have a fine or smooth texture while a loosely woven canvas will have coarse texture (i.e., pronounced weave). A canvas that is tightly woven with a good size thread is likely to maintain its structural integrity. Any canvas with less than 50 threads per square inch / 62.5 threads per 10 square cm is generally not suitable for painting where permanence is the first consideration.

The weight of a canvas is measured in grams per square meter (gsm) or ounces per square yard (oz). The classification according to weight are as follows: light-weight is about 5 oz (140 gsm) or less; medium-weight is about 8 oz / 230 gsm; heavy-weight is about 10 oz / 280 gsm or more. Any canvas that is less than 7 oz / 200 gsm is generally not suitable for painting. There are exceptions to the rule that lighter canvasses should not be used for permanent paintings.

The texture of a canvas affects the final appearance and feel of the painting. The choice of texture depends largely on the medium, style of painting, and preferences. Sometimes, artists choose a rough canvas because they would like to make use of the grid-like weave as a prominent feature in their painting. Other times, artists choose a smooth canvas because they prefer the weave to be unnoticeable. The peaks in the rough canvas are good for creating subtle highlights using a dry-brush technique. A smooth canvas allows the paint to glide and thin out to create the desired transparency and watercolour-like washes. Smooth canvasses are also used to create an even and levelled finish in certain styles of painting. The range of textures from no-grain to extra-rough allows artists to achieve the above results to varying degrees.

Cotton and linen (made of flax) are examples of natural fibres. Linen fibre is brown in colour and cotton fibre appears cream. Cotton canvas has an even mechanical weave appearance while linen looks more natural and irregular. Linen fibres are longer and stronger than cotton fibres; therefore, they are considered more durable. On the flip side, the stiffness of the linen fibres makes it harder to stretch. A cotton canvas is easier to stretch and remains taut for longer than linen; however, linen canvasses hold the shape better than cotton in a large painting. There are many century-old artworks made on linen canvas that remain in a good condition, attesting to its strength and resistance to decay. On the other hand, more time and data are needed to determine the longevity of cotton canvasses.

Polyesters and high-density polyolefin are examples of synthetic fibres. Polycotton is a hybrid canvas weaved with threads that are made of 60% cotton and 40% polyester fibres. The synthetic and hybrid canvasses tend to be smoother, stronger, and considered more durable than natural fibres. Unlike linen and cotton, the weave in synthetic fibre canvasses for the most part is going to be nearly uniform. Polyester and polycotton are far more flexible than cotton or linen, making them easier to stretch. There is also very little slacking with synthetic fibre canvasses.

Alkyd is a synthetic resin that was first developed more than a century ago for industrial use. Alkyd is a term coined from the original name 'Alcid', which indicated that it was derived from alcohol and acids. The use of alkyd as a binder in artist materials such as paint, medium, ground, and varnish is fairly recent. Alkyds used in artist paint and medium are made through a special reaction caused by combining polybasic acid, polyhydric alcohol, and the fatty acid of drying vegetable oil (soy, linseed, safflower). During production, the fatty acid becomes an integral part of the alkyd molecule; consequently, this type of alkyds is often referred to as 'oil-modified alkyds'.

There are many types of alkyds available to manufacturers of artist materials. The fatty acid content in each type of alkyd can vary greatly. Oil-modified alkyds used in artist paints and mediums require at least 60% fatty acid (i.e., long oil alkyds) to prevent the dried paint film from becoming too brittle. Driers and silica are often added to oil-modified alkyds to speed up drying time and give extra body. Generally, alkyds made with linseed oil are the most durable and weather-resistant, but on the downside, they are more likely to yellow than soy and safflower.

The main benefit of using alkyds is the fast drying time and uniformity of palette. A thin layer of alkyd paint (or oil paint used with alkyd medium) will touch dry within 24 hours. Additionally, alkyd paint tends to level more than oil paint and produces a fairly glossy, firm, and durable paint film. Alkyds may yellow slightly depending on the type of drying oil used in the production.

Alkyd-based materials are compatible for use with traditional oil-based materials. Alkyd paints can be intermixed with traditional oil paints and mediums; however, this will slow down the drying time. Likewise, traditional oil paint can be mixed with an alkyd medium to speed up drying time. Alkyd paints and mediums are soluble in mineral spirits or turpentine. Alkyds are sensitive to over-thinning. Too much thinning will lead to loss of adhesion. No more than 25% of mineral spirit or turpentine should be used to dilute alkyd paints or mediums. Once an oil paint is used with an alkyd medium, there should be no going back to a traditional medium; otherwise, there may be problems with adhesion. The fat over lean rule should be observed whether alkyds are used on their own or intermixed with traditional paints and mediums.

The layers of a painting are (from bottom) auxiliary support, support, size, ground, priming, and paint. Each layer has its own function and purpose.

Auxillary Support is the device or framework to which the support is fastened. An example of auxiliary support is the stretcher bars used to secure canvas.

Support is the surface or substrate to which the paint is applied. The support can be flexible or rigid; it's usually made of materials such as canvas, linen, wooden panels, boards, metal, and paper. The role of the support is to hold the paint film in place.

Size is a layer of hyde glue, acrylic polymer, or gelatine applied to the support; it prevents the support from having direct contact with the ground or paint. Size is used to protect the support from the deleterious effects of the ground or paint in oil-based media. Size is not usually applied to supports that hold water-soluble media with the exception of watercolours. Manufacturers size watercolour papers externally with gelatin/starch or internally with Aquapel (a synthetic glue).

Ground is a layer made of white pigment (colour), gypsum (chalk), and a binder (adhesive) applied to the support (or on top of the sizing) to receive and hold the paint. The ground must have the required absorbency, porosity, and roughness. The ground can be water-based or oil-based. Water-based grounds can be used for oil painting but not the other way around. There is a school of thought that considers water-based grounds should only be used for water-soluble media.

Priming is applying a layer of diluted polymer medium (for water-soluble media) or diluted retouch varnish (for oil-based media) to the ground. Priming reduces the absorbency of the ground and enables better manipulation of paint.

Paint is the medium used to create the painting. Examples of media are oils, acrylics, watercolours, gouache, egg tempera, and casein.

(Note: Priming should not be mistaken for the application of ground. The confusion arises because the terms 'Primer' and 'Prime' are commonly used in the product descriptions to refer to ground.)