In the 16th century, artists found better to use fabric stretched. It allowed them to a better mode of transport, a larger size for the artists needed a wooden frame. Strainers and stretchers are the auxiliary support of painting paint on canvas. They are used in other techniques: paper, bark painting and some process of conservation too.

The terms strainer and stretcher refer to the frame (more common is a timber frame) around the canvas is stretched to give a flat surface to work on. It can be reinforced by adding one or more piece of timber on the wooden frame.

The difference between strainers & stretchers

Strainer

The strainer is a frame where all the corners are fixed, they can be stapled, glued, nailed… Strainers are really common to be used by the artists as less expensive than stretchers.

Stretcher

At the beginning of the utilisation of canvas on the wooden frame, artists were mainly using strainers. With time, the canvas comes loose and required to be restretched on a slightly larger strainer. Repeat this action damages the edges of the painting. For this purpose, the adjustable corners have been created.

Therefore, the stretcher is a frame where the corners are mobile, allowing adjustment of tension. It is possible by the presence of wedges (sometimes called keys), small wooden (or plastic) triangles fitting in the join inside the corner.

They push the two bars of the corner apart expanding the size of it creating more tension on the canvas. Antoine-Joseph Pernety (French writer) mentioned about the wedges in 1754 and Watin mentioned them in 1773.

The proportional format starts appearing in the beginning in the 17th century in Holland. In France, they start to use a proportional format before the 19th century and it is still in use today. There are 3 categories based on the different theory: the Portrait will be built on twice the golden section proportion (cf. glossary), the landscape based on the “Porte d’harmonie” rules (cf. glossary) and the seascape proportion based on the golden section.

Principal proportional format for strainer and stretcher used in France (in cm)

Number	Portrait in cm (Figure F)1	Landscape in cm (Paysage P)1	Seascape in cm (Marine M)1
00	16×12	16×10	16×9
0	18×14	18×12	18×10
1	22×16	22×14	22×12
2	24×19	24×16	24×14
3	27×22	27×19	27×16
4	33×24	33×22	33×19
5	35×27	35×24	35×22
6	41×33	41×27	41×24
8	46×38	46×33	46×27
10	55×46	55×38	55×33
12	61×50	61×46	61×38
15	65×54	65×50	65×46
20	73×60	73×54	73×50
25	81×63	81×60	81×54
30	92×73	92×65	92×60
40	100×81	100×73	100×65
50	116×89	116×81	116×73
60	130×97	130×89	130×81
80	146×114	146×97	146×89
100	162×130	162×114	162×97
1202	195×130	195×114	195×97

¹: French translation
²: Non-proportional format

Today, the most common join of the timber bar is a mitred (45′ angle) mortise and tenon joint. This way of assembling the stretcher helps with the wedges to open a few millimetres gap creating a better tension. A squared mortise and tenon assembling can create a distortion in the tension, consequently, some damages might appear. Crossbars improve the stability and strength of the stretcher.

At the end of the 19th century appear in France a bevel on the stretcher to provide future damage.

Since 1970, Australia converted from the imperial system to the metric system however, we can still find products like stretcher which are still in the imperial system.

Imperial (Inch)	Metric (mm)	Imperial (Inch)	Metric (mm)	Imperial (Inch)	Metric (mm)
10	254	31	787.4	52	1320.8
11	279.4	32	812.8	53	1346.2
12	304.8	33	838.2	54	1371.6
13	330.2	34	863.6	55	1397
14	355.6	35	889	56	1422.4
15	381	36	914.4	57	1447.8
16	406.4	37	939.8	58	1473.2
17	431.8	38	965.2	59	1498.6
18	457.2	39	990.6	60	1524
19	482.6	40	1016	61	1549.4
20	508	41	1041.4	62	1574.8
21	533.4	42	1066.8	63	1600.2
22	558.8	43	1092.2	64	1625.6
23	584.2	44	1117.6	65	1651
24	609.6	45	1143	66	1676.4
25	635	46	1168.4	67	1701.8
26	660.4	47	1193.8	68	1727.2
27	685.8	48	1219.2	69	1752.6
28	711.2	49	1244.6	70	1778
29	736.6	50	1270	71	1803.4
30	762	51	1295.4	72	1828.8

Deformations

Tensions

If the tension in the canvas is not perfect, some damages can appear over time. Typical cracks are associated with inappropriate stretch (ladder cracks, garland crack…). An important tension in the fabric can create some tears.

Bevels

A bevel is a surface that meets another at an angle other than a right angle (cf Collins Dictionary)
For instance, if the stretcher is not bevelled cracks in the ladder will appear on the paint layer above the timber bars.

Wood’s weight

When the wood weight is not appropriate to the support, the timber of the frame can twist consequently it will create damages of the support that can be conveyed to the paint layer.