Glossary

ALTERNATE GRAIN DIRECTION - veneers with alternating grain direction make plywood strong.

ASTM - American Society for Testing and Materials Standards. Site

CELLULOSE - carbohydrate which forms the chief component of plant cell walls. It occurs naturally in almost pure form in cotton fiber.

DELTA - triangular section of land at the mouth of a Nile river, in Egypt.

DOUBLE BOILER - also called "Bain-marie" consists of two pans; the bigger in which the working liquid is placed (usually water) and the smaller container (better glass), with the substance that need to be heated. Usually the smaller container is partially immersed in the larger container, and the larger container is heated as shown in fig.1 You can heat the double boiler with electric hot plate, portable clay stove or a gas stove covered by a metal sheet.

If you can't find a glass container that can be only partially immersed, and it will touch the bottom of the saucepan, than you can put between the pans a folded jute/cotton/linen fabric (rag) as it is shown in fig.2 It will make a soft transition of the heat to the glass pot. Never make the double-boiler as it is shown in fig.3 because the transition of heat will be not smooth and it can led to glass cracking.

original double boiler fig. 1	double boiler with rag fig. 2	bad double boiler fig. 3

FRESCO - method of painting on the wet plaster of a wall or ceiling using watercolor/tempera paints.

Formaldehyde - Commercial formaldehyde is produced and sold as an aqueous solution containing 37 to 50 percent formaldehyde by weight. It can be purchased at any drug store and dissolved in water for obtaining the needed percentage (for example in one part of 37% formaldehyde you need to add about 10 parts of water and you'll get the ~4% formaldehyde).

GROUND/PRIMER/GESSO - is a layer, or coat, which is applied to a sized support in order to provide a desirable color, absorbency and surface upon which to paint. The layers must be applied in strokes that are parallel to the edge of the panel, and at right angles one to another; the first layer will be brushed along the long edge and the second one will be brushed along the short edge. Care must be taken to see that each coat is dry before putting on the next. The first coat must be thinnest and leanest. On the layers of the lead paint brush a thinned transparent wash of raw umber. Let the panel sit two weeks or more. Carefully wet sand the surface by hand outdoors until the transparent raw umber wash is completely gone. Some recommend letting the primed panel sit for at least 3 months before painting on it. Never sand the dry surface, lead white is very poisonous. Another important thing is to do the same on the back side of the panel.

LIGNIN - is a complex polymer which combines with cellulose to form the woody cell walls of plants, it's the natural glue of the woods and plants. Llignin must be removed when wood fiber is processed to make paper , using polluting chemicals and a great deal of energy. It is the lignin that causes the paper to yellow.

pH - provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is acidic and less than 7 is basic).

QUARTER-SAWN - Technically, quarter-sawn lumber has the growth rings of the tree approximately perpendicular to the board's broad face. First, we cut a log into quarters. Each quarter is then processed by cutting a single board off of one face, then cutting the next board from the opposite face, and cutting from alternating faces until the quarter is completely cut.

REINFORCEMENT - The cradling of panels, is designed to increase their strength, rigidity, and durability, to counteract their tendency to wrap, curl, or split, and was once used to rectify such defects in the case of old paintings. It was done by attaching wooden strips to their backs with strong adhesives. The classic method consisting of gluing mahogany or oak strips, about 5/8 inch x1/4 inch, to the back of the panel, laying them on the edge, about 2 inches apart for the average panel, and parallel with the grain of the wood, which usually run parallel to the long sides of the picture. A strong hot glue or casein adhesive was used. At intervals of the same distance that separated these strips, they were slotted so that the cross strips could lie flat against the panel. These cross strips were not glued down, but able to move when the panel contracts or expands, otherwise the wood panel may crack. They were usually about half as high as the strips through which they ran. For the large panels it is better to increase the sizes of the strips. The cradled panel was then put into a press and allowed to remain there for at least a week. You can see the cradled panel in the image below.

REMBRANDT - HARMENSZOON VAN RIJN born July 15/1606, in Leiden ( Netherlands ). Died October 4/ 1669, Amsterdam . He was a giant in the history of art, a draftsman a painter and etcher of the 17th century.

Support Induced Discoloration (SID) - that's a term applied to acrylic base products. in our case that's the acrylic gesso (ground) that is applied before the painting layer. The gesso can change its color and thereby effecting the appearance of the colors in the paint film, especially if you are using transparent oil colors.

Research by GOLDEN Artists' Colors Inc. shows that when you'll apply an acrylic base ground/gesso on linen, cotton, fiberboards... (any surfaces that can contain dirt and other impurities), the gesso will change its color.
As the gesso will cure, the water exits two ways: through the surface of the paint and through the back of the support, if porous enough. Canvas, linen, wood and masonite are all porous enough to allow water to absorb into them. During this drying process, the water is actually in equilibrium moving back and forth between the acrylic paint and the support. The water extracts water-soluble impurities such as dirt, sap, starches, etc., from the support and deposits them into the acrylic film. The result is a discolored (typically amber) film, with the degree of discoloration dependent on the amount of contaminants deposited and the inherent level of impurities in the support. Common supports (e.g. cotton canvas, linen, masonite) contain water-extractable materials that can cause discoloration in transparent glazes.

SID contamination often goes undetected. In most cases, the paints applied contain a sufficient level of pigment, thus a strong enough color, to conceal the yellowing.

This support-induced discoloration (SID) can be avoided by thoroughly washing the canvas material with water before use. Or seal the canvas or the board with a thin layer of clear acrylic medium like GAC100 or with a sizing of acid-free PVA.

SIZE - is the first layer in the preparation of painting support. A size is not a coating and not an independent layer; it is a penetrating liquid used to seal, to fill the fibers and to make the support nonabsorbent. If the next coat is composed of substance that can damage or deteriorate the support like oils, it is necessary to seal it, the support will be nonabsorbent and it will protect it. For this reason, we apply a coat of glue sizing (with neutral pH); solutions of animal glue, casein, synthetic resin and so on are used as materials for sizing. It is better to avoid the use of any protein materials such as rabbit skin glue and casein because they're prime sites for mold growth.