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Formaldehyde 8 fl oz

SKU: NAP-500-21FHS08
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US$ 11.83

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Formaldehyde (also called Formalin) is a 4% solution of formaldehyde to harden gesso and chalk grounds and glue sizing on canvas or panels. The spray bottle has a fine misting pump spray. Spray formaldehyde solution onto freshly applied gesso or chalk grounds made with collagen glue (animal or fish glue) as the binder. Formaldehyde is recommended by Kurt Wehlte and Ralph Mayer to harden surfaces made with animal or fish collagen glue.

Directions of Use On Size
  1. Place the support (panel or canvas) upright or vertically.
  2. Spray the formaldehyde lightly over surface of the size after it has been applied and before the priming coat is laid on.
  3. Distribute the coat of formaldehyde as needed to coat the surface evenly using a flat bristle brush or sponge.
  4. Reserve the brush or sponge for this use only. Do not use the same brush used to apply the size.
On Grounds
  1. Place the support (panel or canvas) upright vertically.
  2. Spray the formaldehyde lightly over surface of the final layer of ground, careful to avoid applying excessive amounts that drip down the surface of the ground.
  3. For canvases, spray the front and back of the canvas.
  4. Distribute the coat of formaldehyde as needed to coat the surface evenly using a flat bristle brush or sponge.
  5. Reserve the brush or sponge for this use only. Do not use the same brush used to apply the gesso or chalk ground.
Hardening the Ground

When painting or underpainting with watercolors water-base tempera, any normal absorbent ground will soften or even partly dissolve. The alum content of the size will not be able to prevent this entirely. The ground should therefore be hardened. It is achieved by a six percent solution of Formalin lightly applied to the horizontally placed surface with a soft-bristle brush or rectangular sponge. A specially marked brush should be kept exclusively for this purpose, because any brush that is also used for priming is certain to harden irremediably in time. In order to expose all layers to the action of the Formalin, one can harden the size coating of the support and the final ground layer. However, if lower layers soften during the priming of large areas, causing the brush to drag and making an even application difficult, one may at any time harden each layer as necessary. This takes very little time and is best accomplished while the [sic] support is lying on the floor. It is advisable to leave the room before the gas affects the mucous membranes of the eyes. There is so far no chemical that can reverse the tanning action of Formalin. Once a ground has been hardened with it, it can no longer be washed off and can be removed only with sandpaper.

Hardened grounds naturally show a greater resistance to moisture, even later on, and are thus a major factor in prolonging the life of the painting. Although the practical effect of hardening resembles somewhat that of an isolation when aqueous paint systems are used, the absorbency of the ground is in no way affected by Formalin. Its hardening action is a chemical process that leaves no isolating substance, such as a resin, in the ground. It should also be pointed out that watercolor and gouache, which are bound with vegetable gums, cannot be rendered water-insoluble with formaldehyde. Only animal proteins are affected by its tanning action.

Kurt Wehlte (1975). The Materials and Techniques of Painting. New York: Prentice Hall. pp. 359-60.

The best hardeners belong to class of completely volatile materials (ketones and aldehydes) of which formaldehyde is the most widely used example; the best hardener for casein, glue, or gelatin is a 4 percent solution of formaldehyde, which is brushed or sprayed directly onto the film.

Ralph Mayer (1991). The Artist's Handbook of Materials and Techniques. Fifth Edition. New York: Viking Penguin. p. 487.



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