TECHNICAL INFORMATION

INTRODUCTION
I have assembled here technical notes on painting in acrylics and oils, which will help you to understand some of the properties of these media.

WHAT IS PAINT?
Paints are composed primarily of two components: pigments, the coloring matter, and binders, the media in which the pigment is suspended and which forms a strong film of paint when dry. There are many media used as binders in artist's paints. Before the invention of oil paint, most small paintings in the West were done in tempera, where the pigment is suspended in an emulsion made from egg. Oil paints, are pigments ground in a medium of linseed oil, derived from the flax plant. Acrylic paints, which were introduced in 1957, are pigments ground in acrylic medium, a polymer emulsion thinned with water.

ACRYLIC AND OIL PAINTS COMPARED
Acrylics are recommended for Art 281. However, some students have oil colors, have worked in oils before or simply want to use oils. If you are having trouble deciding which to use, here are their chief differences: acrylics dry quickly allowing an area to be repainted rapidly, but making working wet-in-wet on canvas difficult. Acrylics dry darker than they look when wet, making it harder to judge contrasts between colors. They require no dangerous, spirit-based solvents and clean up handily with soap and water.
Oils dry slowly allowing for long periods of mixing wet-in-wet on canvas, but making repainting of an area to be changed a long-term affair. Oils do not change color appreciably as they dry, making it easier to judge color contrasts while painting. They require spirit-based mediums which produce dangerous vapors, so extra care in their use is needed. Brushes clean up with spirits followed by soap and water. The brushes must be allowed to dry out completely before painting is resumed.

THINNERS AND MEDIUMS
The term medium, in addition to signifying the binder in a particular type of paint as described above, also refers to compounds which are added to artists' paints to change their working properties. Thinners are used to thin artists' paints and to clean brushes and tools.

OIL THINNERS AND MEDIUMS
Oil paints may be thinned with mineral spirits or turpentine, but when painting, do not add so much thinner to oils that they become watery-runny like watercolors. Such dilutions will weaken the paint film. In this class, we use Weber Turpenoid, a low-odor brand of mineral spirits for thinning oil colors. If you want to save some money, you can purchase mineral spirits at a paint or discount store in quart or gallon containers for use in cleaning brushes and tools. Be sure to get "low odor" mineral spirits.
Oil paint media are numerous, but the basic ones are Damar varnish, Stand oil and Cobalt linoleate drier. Damar varnish is used to add gloss and transparency to oil paint, and as a final protective varnish over a dry oil painting. Stand oil is linseed oil that has been polymerized by heating. It is thicker than the linseed oil used to make oil paints and produces mediums that are resistant to yellowing with age. Cobalt linoleate drier increases the drying rate of oils and should be used in very small amounts.

A "standard" medium for thinning oil paints is:
 2 parts by volume stand (or linseed) oil
 1 part by volume damar varnish (5-pound cut)
 6 parts by volume turpenoid
 
A typical glazing medium for oil paints is:
  3 parts by volume of stand oil
  2 parts by volume of damar varnish (5-pound cut)
  3 parts by volume of turpenoid
  1 or 2 drops of cobalt drier per pint of medium

ACRYLIC THINNERS AND MEDIUMS
Acrylics are thinned with water or acrylic media. Tools and brushes clean up with soap and water. When painting, do not thin acrylics with so much water that the paint becomes watery-runny. Doing so will produce poor paint films with low adhesion. Acrylic mediums are various kinds of acrylic used in conjunction with acrylic paints to produce different working characteristics. The chief acrylic mediums are: gloss medium, matte medium, gel medium, modeling paste and retarder.
Gloss medium may be added to acrylic paint to increase its glossiness or used over dry colors to similar effect. If the medium is thinned too much with water it may not be evenly glossy.
Matte medium may be added to decrease the degree of gloss to produce a flatter finish. If applied over dry color, it sometimes leaves a slightly "milky" film.
Gel medium can be added to paint as a thickener where a heavier paint texture is desired. It is generally clear. Adding large amounts will make colors transparent so thick, but still transparent, qualities can be achieved.
Modeling paste contains acrylic medium, white pigment and ground marble dust to give it body. It can be used to build up heavy textures, typically with the palette knife. It can be used under paint or mixed with paint.
Retarder is designed to slow the drying of acrylic paints.

PAINTING SUPPORTS
A support is the surface on which the painting is made. Common supports are stretched canvases, paper and panels of wood or masonite. Each type of support is described below.

STRETCHED CANVAS
A stretched canvas is made from a stretcher, a frame (usually wooden), on which a canvas is stretched and tacked or stapled to create a support for painting. Stretched canvases have withstood the test of time. They are light in weight and very durable. Factory-made stretcher strips, such as those sold in the Art Store, come in two weights. Light milled stretcher strips are good for paintings up to about 30" x 30". For larger canvases, use the heavier ones which are good, unbraced, up to about 45" x 45". They can, with bracing, be used to even larger sizes. The heavy strips are available up to 72" in length. For paintings, over 72" on a side, a home-made stretcher of heavy lumber (usually 1" x 3" or 1" x 4" pine) is needed. Put the stretcher together and check its squareness with a draftsman's triangle, a T-square or carpenter's square. Make sure it is square before attaching the canvas.
Canvas is usually woven of cotton or linen. We have cotton duck of appropriate weight for painting in the Art Store. It is sold by the yard and is available in 72" and 120" widths. To stretch a canvas, cut the canvas at least 2" larger than the stretcher on each side, more if you are working large. Line up the weave of the fabric with the edge of the stretcher. Put tacks or staples in the edge or back of the stretcher pulling the canvas tight as you go.
Begin by putting one staple in the center of one side. Move to the opposite side, stretch the canvas and staple it in the center of that side. Stretch the canvas again, placing a staple in the center of each of the other two sides. Re-check the squareness of the stretcher. Proceed in this manner, working from the center to the corners. At the corners, make a neat fold in the canvas and staple it down. Stretching the canvas this way will keep the stretcher square and the tension on the canvas even.

PANELS
A panel is an attractive alternative to the stretched canvas support for artists who want a very smooth surface or prefer that their support not "bounce" when touched by the brush. Panels are also a good choice if a cut-out or "shaped" support is needed. They can be sawn into almost any conceivable shape with a jig saw prior to painting. Panels are usually made of plywood or masonite, both of which come in sheets up to 48" x 96". Luan plywood, available in 3/8"-thick sheets, or pine plywood in thicker sheets works well as does masonite, which is available in 1/8" and 1/4" thicknesses. Small panels, 15"-20" on a side, can be used without support, but larger sizes will warp unless they are attached to a wooden stretcher. A simple solution is to buy a wooden canvas stretcher and glue the panel to it.

PAPER
Paper, if properly prepared with two or more coats of acrylic gesso, can serve as a painting support. Good quality all-rag papers, such as Rives BFK, work well for painting supports.

PREPARING THE SUPPORT
It is customary to prepare or "prime" the support for painting in acrylic or oils. This preparation keeps the support from absorbing the paint and provides a good base for the painting. While acrylics may be safely applied to unprepared canvas, oil paint will rot canvas or paper if it is not isolated from the support. The simplest way to prepare canvas, panel or paper supports is to prime them with acrylic gesso.

ACRYLIC GESSO PRIMING
Acrylic gesso is acrylic medium with inexpensive pigment added, usually chalk or unbleached titanium. Two or three coats are sufficient to prepare a support. The first coat should be thinned with water-- ca. 40% water to 60% gesso by volume is good. Acrylic medium may be added to increase its flexibility. The gesso should be spread over the surface of the support and worked well into it with a brush or sponge. After it is dry, a canvas or panel should be lightly sanded with sandpaper and dusted off before each additional coat is applied. Paper supports usually do not need sanding. The more coats of gesso that are applied, the smoother the surface of the support becomes.

TRADITIONAL PRIMING FOR OIL PAINTING
Prior to the invention of acrylic gesso, traditional oil technique required a two-stage process of preparation. Many oil painters still use this method. First a coat of sizing is applied to the untreated, stretched canvas. Sizing is an animal-protein-based glue, usually rabbit-skin glue or some other "hide" glue. This is obtained in crystal form, mixed with water and heated to a thin solution, applied to canvas with a brush in a thin but unbroken layer. This layer of sizing serves to isolate the oil paint from the fabric.
The traditional oil priming follows. This is, most often, a layer of white oil paint, thinned with spirits, and applied in a thin layer with a palette knife over the sizing. When the ground is dry, painting can commence. In truth, once the glue sizing is dry, any color of oil can be applied as a base for the painting. The chief disadvantage to this method is that the oil ground should be dry before painting begins. It can take weeks to dry.

SPECIAL PAINTING TECHNIQUES
Collage: when working in acrylic, paper, fabric and other materials may be adhered to canvas using acrylic medium as a cement. Fabric can be soaked in medium and placed on the painting's surface. Paper can be coated with a brushful of medium and stuck down. With oils, damar varnish may be used in the same way.
Hard edge: Sharp edges are easily achieved through the use of masking tape. This technique works best on a smoother surface, on one with several coats of acrylic already on the canvas (rather than working on gessoed canvas, paint over the gesso with a few coats of acrylic paint to seal it.) Put the tape on as desired. Rub the tape down firmly and burnish its edges with the back of a fingernail. To keep color from running under the tape, paint a coat of diluted medium (60% medium to 40% water by volume) over the tape. When this seal is dry, paint in the desired color.
Spray: Acrylic can be applied by airbrush for soft, blended effects. Various masks can be used to control the painted shapes. Always use a respirator (mask) and spray booth when spraying-atomized paint isn't good to breathe.
Stain: Acrylic can be thinned and poured or brushed directly on unprimed canvas to soak in almost like a dye. This technique was used by a number of prominent artists, most notably Helen Frankenthaler and Morris Louis.

CLEANING UP
Clean your painting area and brushes after each painting session. It is far easier to clean up each day than to wait until the paints have dried! If paints have dried hard in brushes or on the palette it may be necessary to use paint remover or "stripper" to clean up, a noxious and time-consuming process.
Acrylics: Cover good paint in cups with a sandwich bag and rubber band. Clean your palette with a little water, scraping it with your palette knife or a razor blade. Wipe off any remaining paint with a damp paper towel. Put palette-scrapings in a scrap of paper and throw them away in the trash. DO NOT PUT PAINT DOWN THE SINK! Wash brushes out with hand soap and water until no more color comes out. Reshape the bristles with your fingers and put away. Since acrylic paints thin with water, brushes need not be thoroughly dry before painting recommences.
Oils: Any substantial amount of unmixed oil color may be left at the edge of the palette for use in the next painting session. Dried paint should be completely removed. Scrape off the mixed paint in the center of the palette with a palette knife or razor blade. This paint should be put on a scrap of paper, wrapped up and thrown away in the trash. The remaining film of paint may be cleaned with a small amount of mineral spirits and a paper towel or rag. DO NOT PUT PAINT OR THINNERS DOWN THE SINK! Brushes should be dipped into thinner and wiped clean until no more color appears. They should then be washed out thoroughly in warm water and soap, have their bristles reshaped and be put away to dry. Since oil colors should not be mixed with water, brushes used for oil technique should be dry before painting is resumed.

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