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.