Grounds for oil painting

 The preparation of canvas
It is normal, but not imperative procedure to stretch the canvas tightly over a wooden frame which is called the stretcher. The universally used stretcher bars of the tongue-and-groove type are made of well-seasoned wood. The advantage of interchangeable bars and the convenience of easy assembly make these stretchers worthwhile. However, when the budget prohibits the purchase of such stretchers a simple alternative may be made from well-seasoned Oregon pine or meranti bars. Using triangular plywood or masonite braces for butt-joined stretcher corners the bars and braces are glued and screwed together. The triangular braces are attached at the rear of the stretcher and the face side should be well bevelled to prevent the taught canvas from knocking against the inner edges of the stretcher.

There are a number of methods for tacking the canvas to the stretcher and the following is perhaps the most common: A piece of canvas about 20cm longer in both dimensions than the stretcher is placed face down on the table or floor. The stretcher is centred on top of the canvas. After the bars have been aligned with the vertical and horizontal weave of the canvas as neatly as possible there will be an excess of about 10cm all round. Drive in a tack or staple at the centre of each bar, using stretching pliers with a strong tension. The canvas will then have a diamond-shaped wrinkle. Drive the tacks or staples in at regular intervals starting on a long side (about six staples or tacks per turn), moving around the stretcher as you advance towards the corners. Use the stretching pliers with strong tension, but not so strong as to wrinkle the canvas between the tacks or staples. Before applying each tack or staple, tug the canvas with your other hand to tighten it horizontally. Finally the corners may be folded in and the surplus canvas may be stapled or glued to the rear of the frame.

1. Stretcher bar. Note beveled edges	2. Joint assembly
3. Assembled joint	4. Completed joint with expansion wedges inserted
3. Place stretcher on top of canvas, checking for squareness and work from rear on flat surface, leaving enough excess canvas for gripping and stretching	4. Grip canvas with stretching pliers and stretch tightly before stapling or tacking. Work from middle of each side towards corners.
3. Ensure that canvas is also tugged laterally between staples or tacks to avoid slackness or gathers between staples	4. Preferably use rust-proof staples or tacks. Do opposing staples/tacks in opposing pairs working your way around the canvas until you reach the corners to ensure even stretch.
3. A distance of between 5-7cms between staples/tacks is recommended	4. Excess canvas can be trimmed or tacked down, but leave enough excess for possible re-stretching later.

Staples or tacks on the side edges or rear of the stretcher? The advantage of driving the nails, tacks or staples into the side edges of the canvas stretcher is that it tends to be an easier operation, especially when you’re working unassisted. But the protrusions of tacks, nails or staple heads along the edge of a stretched canvas can be visually irritating and often causes frustration to picture framers who have to compensate for these irregularities whilst framing.

Applying tacks, nails or staples on the rear surface of the stretcher leaves the side edges clear, which facilitates ease of priming. It is also always advisable to apply your sizing, ground or primer around the corner formed between the front and sides of your stretched canvas. This ensures that oil paint will never spill or bleed onto unsized or un-primed canvas. One of the common problems encountered by art restorers is that of oil paintings literally disintegrating along the front edges of the stretcher. This is caused when painters fail to size, prime or ground the canvas around the corner onto the sides of the stretcher, thus exposing raw unsealed canvas fibres to oil painting media, which causes long term rotting and disintegration of canvas.

The ground
 Unless specially treated, textile fabrics are porous, very absorbent and consume much expensive paint which sinks into the fabric and ultimately imposes severe limitations upon a medium which is normally very diverse. Furthermore the fibres of the fabric absorb the oil from the paint and soon become brittle and with time will disintegrate.

Thus a sizing is required to seal the fibres and reduce absorbency. The best gluesize is made from rabbit skin and comes in thin yellowish sheets, but is increasingly difficult to obtain in South Africa. Gluesize is also obtainable in powder of “pearl” form (pearl-shaped granules) at certain hardware and paint retailers. Such glue is manufactured from the bones and hides of animals, and consists of complex protein which contains two chondrin and gluten; the former responsible for its adhesive properties and the latter for its gelatine.

The most reliable and affordable gluesize available in South Africa is an industrial grade-75 gelatine. It is manufactured for the food industry and compares well with rabbit skin and pearl glue. The more refined forms or grades of gelatine are unsuitable for grounds for painting supports, since the very properties for which we use gluesize, are refined out of the finer grades, which include all cooking gelatines.

For the size, one part of glue is added to about twelve parts of water. When melting the flue in the water a double boiler should be used to ensure that the glue does not boil.  Glue must never boil – it darkens and loses its adhesive properties when boiled.  When the glue is completely melted and dispersed in the water (constant stirring helps) the canvas may be “sized” by applying the mixture to the canvas with rigorous brushing.  Size sparingly.

The following products serve as inexpensive and convenient alternatives to the traditionally prepared grounds for canvas:

“PVA” is the general abbreviation for polyvinyl acetate. A good brand of PVA serves as an adequate ground, and since it contains no oil, there is no need to pre-size the support. A commercial product like Plascon Polvin is readily available and much cheaper than artists’ acrylic grounds.

 “Universal undercoat” is the commercial name for a commercial “oil-based” paint which is used as an undercoat in the building industry. Some painters seem to prefer “Universal undercoat” as a ground, but most find it too oily and unresponsive to oil painting.

“Flat white” is also an “oil-based” paint used as an interior domestic paint. Both of these consist of an oil vehicle stained with chemical agents or synthetically stained pigments. Some painters seem to prefer “Flat white” as a ground, but most find it too oily and unresponsive to oil painting.

The paints in the above-mentioned category are obtainable at most hardware and paint retailers and generally require more than one coat before an adequate ground is established.   Such coats should never be very thick.

The traditional grounds for canvases entail a certain amount of preparation which may be described as follows:

White lead ground can be obtained in the form of a “white lead” paste which is stocked by certain hardware and paint retailers. The paste should be thinned to a suitable degree by placing the “blob” or ball on a smooth slab and mixing in amounts of pure turpentine (not to be confused with mineral turpentine) with a steel spatula.   When the appropriate consistency is reached the white lead may be applied with heavy brushing, working the first application well into the weave. After a thorough drying (4-6 days) the second coat which should not be thinned, is applied with a spatula or brush. The ground requires about six days drying.

White lead is extremely poisonous and skin contact should be avoided. It is advisable to wear rubber gloves while preparing this toxic ground. Never work with lead pigment in powder form unless your procedures conform to the strict laboratory safety protocol required for this highly dangerous and toxic pigment.

The preparation of wooden boards
It appears that the pressed wood board, commonly called “hardboard” (discussed under “the support”), is the most reliable support of this kind, and thus the following preparations relates to this type of board.  The size and grounds to be discussed apply to most other wooden supports.

Hardboard may be cut to the required dimensions by cutting a very deep incision (using a steel rule and a strong cutting knife) along a marked guideline, placing the incision just over the edge of a table and snapping the board by banging the overlap with the hand.   A neat snap should occur if the incision is sufficiently deep.

If any dimension of the board exceeds about 50 cm batons should be glued to the rear edges of the board to prevent warping that may occur in larger supports of this kind.   Well-seasoned Oregon pine or meranti batons suffice and simple butt joints at the corners are adequate.   The glued surfaces should be clamped together with the greatest possible pressure and perfect surface contact until the glue is completely dry. This procedure requires no nails or screws unless the dimensions of the board exceed about two metres. When using clamps, ensure that protective wooden strips are inserted between the board front surface and the clamp to avoid the clamp forcing a dent into your board’s surface. Instead of clamps, heavy weights may be used (working on an even, flat floor or surface) to press the boards and rear batons together.

After a very light sanding of the surface the support is ready for sizing.

The ground (for wooden boards)
The preparation and application of the size is the same as that discussed in section 2.1.2.  The corresponding description of grounds for canvas supports also apply to hardboard supports.   However, the so-called “oil emulsion ground” remains to be described.  This ground has considerable advantage over the previously described commercial products because it is prepared from independent ingredients and thus the exact qualities of the ground may be predetermined.  The basic preparation requires the following ingredients:

Glue solution: 1 part glue to 8 parts water (or even 10 or more parts water depending on the strength of your glue)

Oil emulsion ground
Glue solution (1 part gluesize or grade 75 gelatine to 12 parts water)
Calcium Carbonate and Zinc Oxide added (easily stirred in) in equal proportions until the mixture reaches pouring cream consistency.
Boiled linseed oil between 20% to 50% by volume

The glue (or gelatine) and water are prepared in the same manner as the size.  While the mixture is luke warm stir in the calcium carbonate (which gives the ground its substance and absorbency) and the zinc oxide (which gives the ground additional brilliance).   The addition of boiled linseed oil which should constitute between 20% and 50% of the total volume of the mixture, reduces absorbency and provides a good “sympathetic” ground.   It is important to add the oil in small quantities with constant stirring and beating.   A few thin coats applied with a large brush will provide an excellent ground.   With discretion the amounts of the ingredients may be varied to give different qualities to this ground which is often, but incorrectly is called “gesso”. 

The preparation of paper and card (and grounds)
Paper and card provide an ideal support for grounds for oil sketches and experimental work when permanence is not required.  The grounds used for canvas are suitable for sized paper.   It is not imperative to size and ground paper when durability of the support is of no concern;  and if the paper or card is found to be too absorbent a weak solution of gluesize may reduce absorbency (one part glue and ten to twenty parts water).   When paper is sized the buckling may be avoided by stretching the paper beforehand, in the manner in which water-colour paper is stretched.

 Grounds for acrylic polymer painting
The supports for polymer grounds need not be divided into separate categories as with oil painting.   The supports for polymer grounds may be sized if the absorption of the ground or “primer” by the support is to limited – but a size is not imperative.  The term primer is used because polymer primers do not require a size, and are therefore neither “size” nor “ground” but serve as both at once.

Polymer primer is a white paste or paint composed of titanium white and other white or inert pigments dispersed in an acrylic polymer resin or an acrylic-vinyl polymer vehicle.   Although many brands are labelled “gesso”, primer has no significant properties in common with real gesso and cannot be used as its substitute.   The primer is applied in several thin coats (thinned with water if necessary) until the desired ground has been established.

The better commercial types of PVA previously discussed serve as inexpensive substitutes for the polymer primer which is stocked by art material retailers.

Preparation of water-colour supports
Water-colour paper requires no ground and is generally prepared and sized during its manufacture by the maker.  Naturally, paper is extremely absorbent to liquids, and in order to manipulate liquid paints or inks upon it, it must be impregnated with sizing.  An example of unsized paper is a blotter; another is filter papers, the best grades of which are the purest form of paper, and are often products of the same makers who produce our water-colour papers.

The material used for sizing good water-colour papers during their manufacture is a weak solution of gelatine or hide glue, and the amount used is of considerable importance to the properties of the paper. Paper which has been made with too much sizing will give irregular or spotty results and will feel resistant to the medium, while that which it too absorbent appear dull with the loss of chromatic distinction causing the medium to draw into the paper in a blotchy manner without the clear edges to colour that is so typical of the medium.

Chinese and Japanese papers made in the traditional manner, perhaps with few open improvements, are sized with a solution of ex-hide glue, which is hardened or set with alum.   Some kinds of Japanese papers are sold unsized and ink or colour will spread uncontrollably on this material unless it is impregnated with size before use. This paper is sized by immersing the sheets in a weak solution (less than 7g gelatine to 5 litres water) of gelatine.

Water-colour paper is stretched by immersing the paper in water and then flattening the paper on a clean and damp board (which must not warp when wetted). Wet strips of gum-tape are flattened along the edges of the paper thus sticking the paper to the board, and bubbles which may have framed between the board and paper to the board, and bubbles which may have formed between the board and paper are gently squeezed out with a sponge or soft cloth.  The paper is then left to “stretch and dry”, when it is ready for working.

When stretching water-colour paper, avoid rubbing the gum-tape in such a manner as to spread the gum onto your paper, as this (the gum) will act like a sizing on your paper, and cause uneven absorption of the medium; also yielding unpredictably blotchy results whilst painting.