Painting Supports

(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)

TODAY IT IS RARE to find artists making their own brushes or paints; however, the reverse is true when it comes to preparing a surface or support to paint on. An artist’s involvement in preparing a painting surface may range from purchasing raw canvas, stretching it, sizing it, applying a ground, and priming it, to simply apply­ing a second coat over preprimed stretched canvas.

Preparing your own surfaces to paint on provides the opportunity for you to create a working surface that meets your own requirements. Manufacturing it yourself also saves money. Too often, however, an artist’s impatience, lack of money, and incomplete understanding of the role of a properly prepared painting surface results in a painting that eventually requires major conservation or simply self-destructs. The unfortunate reality is that the preparation of a proper painting surface is too often the last consideration that a painter makes.

There are some basic guidelines for the preparation of a painting surface that have developed over the last seven hundred years. If these guidelines are vio­lated, disaster will result. This does not mean that only traditional surfaces may be used, but rather that the principles of these guidelines must be understood so that new, as well as old, surfaces can be properly prepared so that artwork will have a reasonable chance of survival through time.

Portable Supports

There are two types of portable supports-stretched and unstretched. A stretched support is a material, such as canvas, stretched over a frame or panel. Unstretched supports are plywood, fiberboard, particle board, metal, paper, board, and even canvas without a frame. The purpose of having a portable sup­port is to be able to move it easily from place to place. Therefore a support’s most important characteristics are that it be lightweight, stable, and durable. Often these factors must be balanced among one another. For example, if the support is too lightweight it is often not stable. And, if it is made to be very durable it is often too heavy. Each situation is unique and requires its own balancing act.

Auxiliary Supports

The purpose of an auxiliary support is to give structure and support to the sup­port itself. An auxiliary support may be either stretcher bars, a frame, or a panel. A panel is a uniform structure with at least one solid, flat, unbroken surface. The first panels were made of wood planks butted against each other, the surface of which was used to paint on. Today, lightweight panels are used as auxiliary supports as well as supports. A frame consists of a set of bars that are fixed together and cannot be expanded. Frames were first seen in the seventeeth ­century Dutch school of painting. Their introduction was a major advance in reducing the weight and increasing the portability of paintings. Stretcher bars are bars designed to produce a frame that can expand slightly to tighten further a stretched canvas. The first stretcher bars with mitered corners and keys (little wedges used to force open the corners of a stretcher bar frame to tighten a stretched canvas) appeared in the early nineteenth century.

Frame Supports are the simplest and the oldest auxiliary support for stretching canvas. They are the easiest to make, but they have no provision for tightening sagging canvas. They consist of four pieces of wood in small sizes, which are simply glued and nailed together. Cross braces are added for larger sizes. Today, frame supports are made from kiln-dried pine or mahogany, which has less tend­ency to warp. The bars are usually made from 1″ x 2″ lumber that has had one of the 1-inch sides milled at an approximately 30-degree angle to produce a ridged edge. The ridge allows the canvas to be stretched with only one edge of the bar touching it, rather than a whole side, which could leave an unsightly impression on the front of the canvas.

The corners and cross braces are often reinforced with triangles, made of 1/8” – 1/4” -inch plywood, nailed and glued to the back. All wood should be seasoned, cured, or kiln-dried and should be free of knots. Cross bracing is most often done every 18 inches. With heavy bars made from 2″ x4″ wood, cross bracing is done every 36 to 48 inches.

A Framed Panel Support is a piece of lightweight mahogany veneer plywood with a frame attached to the back to give additional reinforcement to prevent warping. Panels such as these are used to stretch canvas over. The panel is made from plywood ‘/s-inch thick, or ‘/4-inch thick for larger sizes. The frame is made of l” x 2″ (milled 1″ x 2″ is actually ¾” x 13/4″) mahogany strips with cross bracing every 16 inches. The panel is glued to the frame and sealed with a wood sealer. The advantages of this type of auxiliary support for stretched canvas is that it is strong, yet lightweight, and it allows for the easy sanding of a gessoed surface to achieve a smooth finish. It also provides support for drawing and for egg tempera paintings. The problem with this kind of rigid support is that the canvas cannot be tightened without restretching. In addition, care must be taken not to glue the canvas accidentally to the surface of the panel when sizing it or when preparing a ground.

Stretcher Bars are designed so they can be assembled without being fixed in place, and the comers can be expanded after the canvas has been stretched. They should always have a raised lip at the outer edge to keep the canvas from touch­ing the inside edge of the stretcher bars and leaving an impression. The main purpose of stretcher bars is to remove any sagging of the canvas-a recurrent problem with linen-which can occur during stretching and sizing, as well as during the application of the ground. Expanding the corners of a painted picture is not wise because small cracks can develop, which will eventually become larger, especially in older paintings. The general rule is that if a sagging painting cannot be corrected with minor adjustments, leave it alone or get help from a professional restorer.

There are several types of corner designs-tongue and groove, Lebron, Muleski, and Starofix. The tongue and groove system is the style most com­monly used for commercially made pine stretcher bars. They range from 5/8 to 11/16 inch in depth and 1 1/2 to 1 5/8 inches in width, and lengths from 6 to 72 inches with increments of one inch. One end of each bar is cut so that it will slide into the groove cut into the end of the other bar. The cutting remnants, which are in the shape of little triangles, are used to expand the corners by wedging them into the gaps in the inside of the joined corner after the canvas has been stretched over the bars. A heavy-duty version of these bars can some­times be found in art supply stores or can be ordered through them. One of the best I have seen is manufactured by the Best Moulding Company and is illus­trated opposite.

  

Two types of hardware are used to expand the corners of stretcher bars. The first type is a turnbuckle, which involves a metal link that holds together a screw on each end. The screw has an eyelet at the end so that it may be fastened to a surface. The screws have opposite threading so that when the link is turned the screws are pulled together. The Muleski stretcher bar corners, named after the designer Thomas Muleski, employ turnbuckles.

The corners have a large groove in which a small aluminum plate is inserted and the eyelet turnbuckles are fastened to the inside of the corner groove. The link is exposed in the inside of the corner and when turned the corner expands along the aluminum plate.

The Lebron stretcher bar system, named for the designer James Lebron, uses a Tite-Joint Fastener, which consists of a locking sleeve on one edge of a draw bolt and a tightening nut assembly on the other end.

The nut assembly and the locking sleeve are embedded into each side of the corner. Dowels are placed on each side of the draw bolt, parallel to it. The corner is expanded by turning the round nut with a pin or nail, which fits into small holes made for this purpose. As the comer expands the dowel maintains the alignment of the corner.

A new development in stretcher bars is the use of extruded aluminum strips with outside edges made of wood. A company called Starofix has developed what it calls a “continuous tension stretcher.” A type of adjustable spring clip is used in the inside corners to keep the canvas stretched taut. They are light­weight, and so strong that cross braces are not required for lengths less than 72 inches. They are ideal for conservation work and for very large canvas works. However, they are expensive and their high price puts them out of reach for most painters. In addition, they are not readily available outside New York City.

Stretched Supports

By the Renaissance, fabric was widely used as a support for painting and had almost replaced the wooden panel. Panels are heavy and the paintings on them are easily damaged. A bumped panel can send ripples through the surface, weak­ening the bond between the ground and the support. Fabric stretched over a frame resolved the two major problems of weight and elasticity.

The time-tested practical approach to painting on a fabric support, such as canvas, is to stretch it across a frame, stretcher bars, or a panel. Paintings particularly oils-that are completed on unstretched fabrics and later stretched, form tiny, almost invisible, cracks in the paint film, which eventually become larger. Therefore, unstretched artwork should remain unstretched and stretched work is best left alone.

Of all the possible fabric supports, canvas is the best choice because of its long history of durability. The term “canvas” is derived from canabis, also known as hemp, which, with flax, was the first fiber used to produce sailcloth. “Canvas” now includes jute, which is used to make burlap, and cotton. Of these three, cotton and flax, or linen, are the most popular.

There are several ways to classify canvas. For the artist,. the most important information involves the ounces per square yard and the thread count. This infor­mation, however, is sometimes difficult to obtain. Ounces per square yard will tell how heavy the canvas is. Threads per square inch will tell how dense the canvas is. Canvas that is less than seven ounces per square yard with a thread count of less than fifty threads per square inch is too lightweight for use as a support for painting. It will not be sufficiently strong to resist tearing and the spaces between the threads will be too large to fill adequately during sizing. As long as the canvas is heavier than seven ounces with a tighter weave than fifty threads, your choice can be based more on appearance and feel than technical specifications.

Linen has been found in Stone Age remains and in the tombs of ancient Egypt and has proven its durability. The linen wrappings from Egyptian mummies are still flexible today. The source of linen fibers is the flax plant, whose fibers are stronger than any other natural fiber and range in length from 10 to 36 inches. Linen fibers are round, not flat like cotton, which gives linen fabrics their irreg­ular texture. Chemically, the fibers are 70 to 80 percent cellulose and contain the same oil that is found in the plant’s seeds and that is used in linseed oil. The natural content of linseed oil preserves the fibers and keeps them flexible. On the other hand, it was long ago discovered that paintings using linseed oil on unsized linen rotted quickly, because the fibers can stand only so much linseed oil. When linseed oil dries it becomes acidic and will attack the cellulose. Today, the problem of acidity is greater, because of the acidic air pollutants, which can also attack the cellulose of linen. Bleached linens should not be used as a support for painting because the bleaching process severely weakens the fibers.

The ropelike quality of the linen fibers produces an irregular texture which is especially ideal for figurative painting because of the sense of depth it gives to the surface. Unfortunately, linen is more difficult to stretch than cotton canvas, for it has a tendency to sag after the first attempt, thus requiring tightening. In addition, linen is often two to three times more expensive than cotton and is difficult to obtain in weights greater than seven ounces with thread counts greater than fifty. Although a thin linen canvas is stronger than a thin cotton canvas, artists should nevertheless use linen heavier than seven ounces per yard and denser than fifty threads per inch, with as few knots as possible. Today, the best grades of linen are produced in Belgium. Yet, few artists can afford an eight- to ten-ounce, tight-weave Belgian linen in widths larger than 54 inches, let alone find it. At the present time, most of the affordable linen is imported from South­east Asia. The quality and consistency is not as good, but it is adequate.

Cotton, like wood and linen, has been in use since prehistoric times, particularly in the Orient. The best cotton comes from the east coast of the United States; the second best is from Egypt. Cotton fiber is almost pure cellulose and is easily affected by acid and bleach. Therefore only unbleached cotton fabrics should be used as supports for paintings, and they should be sized before oil painting to protect against acidity.

There are several negative aspects of using cotton as a support for painting. For figurative painting, cotton has a less interesting texture than linen. It is also less strong than linen, for its fibers are shorter, ranging from 5/16 to about 1 1/2 inches in length. Cotton fabrics derive their strength more from the process of twisting the yam during manufacturing than from the weight or thickness. A well-twisted yarn can be stronger than two yams of the same thickness that are not twisted. But, it is very difficult for most people to perceive this quality by casual inspection. What you can look for to give you an indication of the quality is the “trash” content. Trash is the waste recovered from the processing of the cotton bolls and, to cut expense, a small amount is often added back into the cotton as a filler. If too much trash is used, the strength of the fabric will suffer. An indication of the amount of trash in a fabric is the brown specks, called moats, which come from the seeds and husks of the cotton boll. The fewer moats you see, the less trash and the better the quality of the canvas. Lack of strength in a cotton canvas can be overcome by selecting a heavier weight and a tighter weave. Eight ounces per square yard should be a minimum. Most profes­sional painters use a ten- to twelve-ounce canvas with a weave so tight that few if any holes can be seen when the fabric is held up to the light.

Cotton is not as permanent as linen because it lacks linen’s natural oil content. Cotton fabrics are basically unprotected cellulose, which is easily attacked by acidity. The exposure to the acidity from air pollution and ultraviolet light over several decades will substantially weaken a cotton canvas. This can be somewhat compensated for through proper sizing with acrylic mediums.

Commercially Prepared Canvas is an alternative for those painters who do not wish to prepare a painting support themselves. Preparing a traditional glue-sized, lead-primed, linen canvas requires skill as well as effort, and a prepared canvas is often the only sane option.

Manufacturers of prepared canvas refer to their products as “primed” canvas because they are ready for use without further preparation. This is not an accu­rate use of the term “primed,” but it has been adopted because of convenience. It is simply easier to say “double-primed linen,” for example, than “linen with a hide-glue size and a lead white, oil ground.” Unfortunately, this use of the term “prime” has carried over to common artists’ vocabulary, occasionally leading to incoherent discussions of various products and methods used in the preparation of canvas for painting. In this discussion of prepared canvas, the commercial descriptions of products are used, but please do not confuse them with the proper use of the term “primed.”

There are four basic varieties of prepared canvas available in most artists’ materials stores. They are acrylic-gesso-primed cotton, acrylic-gesso-primed linen, glue-sized single-primed linen, and glue-sized double-primed linen. All of them can be used as supports for oil paintings. Acrylic or vinyl paints, however, may only be used on the acrylic-gesso-primed canvases, which conforms to the rule of not using a water-based paint over an oil-based surface. “Single-primed” and “double-primed” refer to the number of coats of lead paint that have been applied. The more coats, the smoother the surface, the stiffer the canvas, and the greater difficulty in stretching it. “Double-primed” is less flexible than “single,” which makes its permanency more questionable.

Commercially prepared primed canvas comes in several forms: rolls, panels, pads, and stretched. The rolls are generally of 6-yard lengths with widths rang­ing from 45 to 84 inches. Most are made from eight-ounce canvases, although different weights and weaves are common. There are differences in quality, but they are, in most cases, not related to permanency, and choice should therefore be based mainly on cost and personal taste. The only caution to be taken in your selection is the weight, or strength, of the canvas. Remember that the larger the painting, the heavier the canvas should be.

Preprimed and stretched canvases are often of a quality too poor for profes­sional work, and are normally reserved for studies or experimentation. The better grades are considered by many to be minimally acceptable in sizes up to 24″x30″. Beyond this size, substantial reinforcement is often required. Canvas pads and canvas panels are not suitable for truly professional work.

Stretching Canvas

Fabrics like canvas are stretched to the maximum during the manufacturing process and this tension has to be maintained, if not increased. If this is not done the fabric will attempt to return to a more relaxed state as the humidity changes or if water-based paints are applied to it. The result can be buckling, sagging, or distortions of the painted surface. It is therefore best to fasten a canvas securely before painting.

Fredrix Artist Canvas, Inc. offers instructions for stretching canvas; this is a simplified version of its recommendations, with some modifications.

Check the squareness of the auxiliary support, and verify that it is adequately reinforced. It can then be placed over the canvas, aligning the vertical and horizontal weave. There should be at least three inches of excess material on all sides so that the canvas can be held and wrapped around the edge of the auxiliary support.

Wrap the canvas around at the center of one of the longer sides of the auxiliary support and then tack or staple it to the back of the support. The opposite side of the canvas should be pulled (canvas pliers are often helpful at this point) and tacked down in the same fashion. Repeat the procedure for the centers of the adjacent sides. At this point, there should be a diamond-shaped wrinkle.

Return to the center of the long side. Choose a point one or two inches to one side of the point where the canvas is already tacked, and pull the canvas and secure it as before. Working outward toward the corners, tack down the oppos­ing side, followed by the adjacent sides. The diamond wrinkle should disappear as the corners are reached. About two inches from the corner, tuck, fold, and tack the canvas in place.

If you are using stretcher bars as an auxiliary support for raw canvas and there are minor wrinkles, the corner may now be expanded to remove them. If the canvas is fixed over a frame or panel, all or part of the canvas must be restretched to remove wrinkles. With raw canvas, dampening and drying will remove most minor wrinkles, but they may return when the canvas is prepared for painting.

Wrinkles in prepared canvas should not be removed by expanding the corners or by dampening, both of which can cause cracking and weakening of the bond between the support and the ground. Care should also be taken not to stretch lead-primed canvas too tightly because fine cracks can develop.

Unstretched Supports

Wood is the oldest unstretched support, and examples of its use date back to 2900 B.C. in Egypt. During the Renaissance, oak was the wood of choice, fol­lowed in popularity by pine and then poplar. Wooden supports were panels made from boards joined together at the edges. Today, even if you chose to duplicate these panels and had the carpentry skills and knowledge of woods necessary to produce them, you would have great difficulty finding properly cured hardwood milled in the correct manner. Panels that are built up from boards joined side to side have the drawbacks of being heavy and having a tendency to develop a washboard effect on the surface of improperly prepared panels. Today’s unstretched supports include canvas, paper, plywood, fiberboard, particle board, and metal.

Plywood is available in two types. One has a solid wood core and a wood veneer on both sides. The other, which is most often used as a support for painting because of its availability and lower cost, consists of several veneers of wood glued with the grain at right angles to each other. The veneers of wood are produced by rotary cutting; a whole log is cut on a giant lathe by rotating it against a blade, shaving off layers. In all veneer plywood the inner layers are of a cheaper grade and are thicker than the veneer. The outer veneer can range from a construction grade to that of a fine-quality hardwood. A hardwood surface veneer plywood with one good side that is at least -1/4-inch thick is sufficiently warp-resistant for use as a support. Plywood is actually stronger than its equivalent in wooden planks and less subject to shrinkage and warpage.

The problem with using plywood as a support for artwork is that the glue that bonds the plies can fail. In addition, a washboard effect can develop in a paint­ing because the veneer is cut in a rotary fashion and then flattened and joined edge to edge to cover the large surface. In general, wood does not expand or shrink equally in all directions and, because of this, not only can this washboard effect appear but the entire board, no matter how thick, can warp. Gluing 1″ X 2″ strips of wood to the back of the plywood and sealing both sides with a wood sealer can help to prevent this.

Sealing plywood also helps to reduce the tendency of the grain (the correct term for what most people call grain is actually “figure”) to swell when such waterborne materials as acrylic gesso are applied. Most commercial wood sealers are oil-based so that they themselves will not swell the grain (figure) of the wood during application. Yet this does not present a problem for the subsequent application of acrylic emulsion paints or grounds because sealers work by soak­ing into the wood rather than sitting on top of it; they do not prevent paint or ground from also soaking in and establishing a firm mechanical bond. This appears to be one of the few exceptions to the rule of never applying a water­borne paint over an oil-based material. The surface should be lightly sanded after the sealer has dried and before the application of the size or ground. This increases the surface tooth for better adhesion.

Fiberboard, commonly called by its trade name, Masonite, is made of ground wood chips, which include bark, that have been broken into fibers with the aid of steam and pressure. The fibers are glued together and formed into a sheet using the lignin content of the wood (lignin is the natural adhesive that holds a tree together) and some synthetic resin adhesive. Wax, rosin, paraffin, and a preservative are often used as additives to control moisture content and to resist attack from mildew and rot. A hardwood fiberboard, commonly referred to as tempered fiberboard, is produced by treating fiberboard with heat and impregnat­ing it with oil. Tempered fiberboard is dark brown and has two smooth sides. Untempered fiberboard is chestnut brown and has only one smooth side. Fiber­board is dense, hard, durable, and does not warp or bend easily. Its thickness ranges from ‘/e to ‘ inch; ‘/a-inch is most commonly used as a support. Sheet sizes can be found up to 4′ x 16’.

Only untempered fiberboard should be used as a support. Tempered fiberboard is unsuitable because the oil impregnation does not allow for a secure ground. Fiberboard supports of 24″x36″ and larger should be reinforced by gluing a framework of 1″ x2″ wood strips, made of either basswood or mahogany, to the back, as would be done for a framed panel.

Particle Board, which is made from byproducts of milled lumber, began to be manufactured during the 1940s after synthetic resin glues were developed. Particle board, an inexpensive alternative to plywood, is formed by mixing wood chips and sawdust with a water repellent and a preservative and then gluing them together. Particle board looks like a wooden version of meatloaf, and when it is made wet it develops a similar texture because the wood chips swell and rise above the surface. Consequently, it is necessary to seal particle board with a wood sealer prior to preparation for painting. It also has no internal structure and tends to sag, particularly when it is exposed to moisture, unless it is sealed and reinforced with wood strips on the back. Particle board comes in the same sizes and thicknesses as plywood. A minimum thickness of 34 inch is best for use as a support for painting.

Particle board is heavy, however, and the edges tend to crumble, which does not make it the best choice for a support. It has also been found to give off hazardous formaldehyde vapors. The only reason particle board still remains in use is because it is inexpensive.

Metal was not used as a support for large paintings until the 1970s. In ancient times, metals were rare and too expensive to be used as a support for painting, except in the case of miniatures. Since metal is heavy, only thin metal sheets could be used and they were often too flexible. Copper was commonly used for miniatures until the development of aluminum, which was first used commer­cially in 1886. It was not a practical alternative until the introduction, in the 1970s, of aluminum honeycomb support panels, which were originally used by conservators for relining canvases. Frank Stella was the first major contemporary artist to use this material to produce sculptural paintings. Aluminum honeycomb support panels have a honeycomb aluminum core with a cell size of 3/8 inch and thickness of 9/16 inch covered on both sides with a skin of aluminum alloy that is 0.025-inch thick. The skin and honeycomb are bonded together with heat and pressure-cured epoxy. Some manufacturers, such as the Process Materials Corpo­ration, supply the panels with a redwood edge. The overall thickness is 5/8inch. There is no standard size because the panels are custom made. They are extremely expensive; a 40″X60″ panel costs several hundred dollars.

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The main advantage of this material is that it does not warp, is very strong and lightweight, and resists sagging. Unlike copper, which reacts with the lin­seed oil in oil paint and turns green, aluminum is relatively inert. However, an aluminum surface that is polished is not as good a support as a surface that is sanded or slightly oxidized. Although all metal supports can be painted on with­out sizing or ground, the surface of aluminum must be completely covered with paint or protected with a varnish. If this is not done, the moisture in the air will react with the aluminum and will result in corrosion.

Unstretched Canvas has been successfully used as a support, but it has never quite caught on. Ed Moses, the founder of nonrigid, unstretched artwork, devel­oped a means of impregnating paper or fabric art with an acrylic polymer medium to give it strength and durability. This process basically involves coating both sides of the material with a pure acrylic polymer (made by the Rohm and Haas Company), or any other acrylic polymer medium. The wet, coated material is then spread out on a sheet of glass to dry. When it is dry it can be carefully peeled off the glass. Collages can be made by adding coated dry material to the coated wet material on glass and, when dry, can be removed as one piece. The surface, although slick, will accept thin layers of acrylic or oil paint.

Unstretched canvas can also be primed with acrylic gesso and then painted over with acrylic paints. However, uneven surfaces, shrinkage, and how to hang, display, and store unstretched artworks make this type of support problem­atic. A traditional hide-glue sizing and lead white priming should not be attempted on unstretched supports because of the potential for cracking.

Paper painting supports should, in general, be heavyweight and have a high cotton content, like 300 lb., 100 percent cotton watercolor paper or 100 percent cotton museum board, both of which are durable and sufficiently absorbent to be adequately sized and primed for oil or acrylic painting. Wood-pulp papers or boards can be used if they are first thoroughly coated with an acrylic polymer medium on both sides. However, these are still not considered as safe for fine artwork as cotton papers and boards. Lightweight papers as well as print papers can be used, but they are best avoided until after experimentation with heavy paper and board. With rare exceptions, when using paper or board as a support the finished painting must still be framed behind glass, like any other paper artwork for proper protection.

Fixed Supports (Walls)

THE SCOPE of this book is limited to painting and drawing materials used to produce portable, and, as much as possible, permanent artwork. Mural painting is, with rare exception, neither portable nor particularly permanent. Although most of us like to think of the walls of our homes and buildings as permanent, weather, sun, heat, cold, acid, alkali, air pollution, earthquakes, wars, fires, bulldozers, and interior decorators have all contributed to their impermanence as well as to the impermanence of any artwork applied to them. In fact, today’s muralist counts himself or herself lucky if an outdoor painting survives for ten years. Examples of indoor murals that have lasted for centuries are due to extraordinary care and protection, and have often undergone conservation and restoration several times. It has recently been discovered that many murals currently under restoration have been so poorly “restored” several times in the past that some repainted areas bear no resemblance to the original painting.

Murals become a fixed part of the environment and, for the most part, cannot be protected. Paper artwork is framed and protected from the environment. Oil or acrylic paintings are executed on grounds and supports chosen for their per­manence and portability. They are also varnished to protect the surface and are kept away from the surface of the wall when hung. Murals are attacked from both the front and the back by the environment.

To produce a mural that will last several decades outdoors, and several more indoors without extensive protection and care, requires an extensive knowledge in several areas, some of them well beyond that of artists’ materials. Assuming the artist knows which pigments have the highest lightfastness rating and are alkali-proof (modem walls tend to be highly alkaline), extensive knowledge of building materials and their chemistry is also required. For example, it is not uncommon for an inexperienced muralist to find a painting on a new concrete wall peeling off after several months. This is because the form oil (used to prevent the wood moulds from being cemented to the concrete after it has set) had not been removed before painting. This oil can stay in the wall for several years and must be removed (usually by sandblasting) before painting begins. It is knowledge such as this that cannot be left to trial and error. Anyone who wishes to do serious mural work should apprentice with an experienced muralist.