The Preservation Handbook Online

Table of Contents

I. Preservation
II. Assessment
III. Collection Assessment
IV. Agents of Deterioration
V. Monitoring collections
VI. Materials
VII. Storage
VIII. Conservation
IX. Disaster Planning
X. Housekeeping
XI. Resources / Suppliers
XII. Preservation Grants

Materials - Painting

Painting - introduction

Paintings are composite objects that respond continually to changes in temperature and relative humidity. Paintings are found on several different supports including: paper, ivory, glass, silk, black lacquer, metal, ferrous metal, copper, plaster, brick, wood, panel, cotton, and canvas. Some paintings will have an auxiliary support, such as stretcher bars; a ground; at least one layer of paint; and sometimes a varnish. Damages such as flaking, cupping, and planar distortion occur over time. Varnishes will discolor and pigments will fade. Controlling environmental conditions, to provide stable, safe, clean, secure, and accessible storage are the most important aspects of painting preservation.

Painting - identification

Period and palette are important indicators for accurate identification. How materials are made and how they are handled will help to identify period, palette can help identify artist or school. For example methods of paint making have changed over time. Modern paints have very fine pigments, while older paints have pigments that you can see. Insight into artistic process and medium will ensure safe storage and ethical treatment for historically significant objects.

Painting - process

Paint consists of 3 components: pigment, binder and vehicle

Encaustic - pigments suspended in hot beeswax; produces rich colors; very stable.

Tempera - pigments suspended in egg yolk, water is the vehicle. Egg yolks can suffer fatty acid migration and thinly painted surfaces become transparent.

Watercolor - pigments suspended in water; gum arabic is vehicle; staining technique applying thin washes of pigment; light sensitive, requires UV protection.

Gouache - opaque water colors.

Buon fresco - finely ground pigment suspended in water and applied to damp lime-plaster surface. Lime in the presence of oxygen forms calcium crystals that chemically bind the pigments to the plaster. Pigment intensity increases with age.

Secco fresco - Tempera paint applied to dried lime-plaster or dried buon fresco.

Oil - pigments mixed with various vegetable oils, such as linseed, walnut, and poppy seed. Dries slowly allowing for great flexibility

Crafting a panel is a lost art and most modern painters use modified processes that create unstable objects. Panels should have sufficient support to hold the paint. This requires a gesso layer on top of the support to act as a solid surface for the painted image. Following the gesso is the underdrawing, underpainting, and paint. Some artist will finish the painting with a sealant or varnish. Several things can go wrong within these three layers that will effect the longevity of the painting.

Supports that have been improperly stretched will subject the painting to stress during humidity and temperature fluctuation that can lead to corner draws and twisting of the canvas.

Gesso can be made with different grounds that affect the flexibility and rigidity of the surface. Gesso made with "lead white" will crack and fail to interplane easily leading to a shifting support and eventual flaking.

The standard rule of painting is fat on top of lean and when reversed the paint will produce "alligator cracks". Paints made with unstable pigment will change colors and fade. Painting on top of wet paint also compromises the structural stability.


Painting - deterioration

Painting's vulnerabilities:

-accidental damage
-aging materials
-environmental effects
-inherent vice
-inconsistent retouching
-incompatible materials
-cleaning and maintenance

Cracking occurs in the varnish layer only

Drying or Traction occurs when the paint layers crack and varnish is applied afterward.

Flaking can consist of both ground and paint or paint alone and may indicate several conditions. The painting may suffer from inherent vice due to unstable materials; artist technique; deterioration of glue size in the ground; rapid changes in environmental conditions; water damage; physical damage; and intense heat.

Lifting - paint has lifted away from canvas but is still attached.

Blind cleavage - paint is lifting, but has no open edges.

Due to the complex structure of paintings, treatment should be handled by a conservator. Treatment may involve the infusion of a protein based or synthetic adhesive in the flaking area. Occasionally wax is used to pinpoint flaking.

Painting - canvas

Tears in the canvas should be treated by a conservator immediately. Once a canvas is torn, distortions begin and increase over time making successful repairs less likely. Large tears will require a new lining.

Quilting occurs on the back side of a painting when the weight of the media deforms the canvas. Can be treated with heat and relining.

Pentimenti happens when artists paint over the top of an image as an afterthought or correction. Over time the pigments in the top layer of paint become transparent and the underpainting shows through.

Patina is an acceptable form of dirt and should not be removed. It allows you to travel through time and experience the piece as it was executed.

Old Master's Glow - yellowing varnish a controversial issue in conservation communities. Removing yellow varnish is highly invasive and alters the natural state of the painting. Removing the varnish changes the character of the painting and is not a reversible treatment, so can not be considered a conservation effort.
Blooming in varnish is where scattered light appears white. This phenomenon indicates conditions of high humidity and temperature. Blooming occurs in Mastic resin but does not happen in Damar resin.

Cleaving, dents and bulges can be treated with humidification.

Mechanical cracking is usually caused by age and penetrates the varnish, paint layer, and ground.

Cupping or curling of varnish, paint, and ground, pulling up canvas to conform to cracks. Can be treated with humidity

Painting - panels

Panels should be stored in microclimate boxes to slow deterioration from inherent vice and reactivity. Panels from Northern Europe were usually made from mahogany, one of the more stable woods, yet still produces acetic acid which accelerates deterioration. Fat on lean paint techniques will encourage paint to crack.

Paintings on rigid supports are considered to be at greater risk in transit than canvas.

Painting - storage

Acid-free backing board screwed to the verso of the stretcher bar or frame is the single most important preventative conservation step. The trapped air cushions the paint. Paintings should be examined with raking light for cleaving paint. Good condition - store vertically. Bad condition - store horizontally. Manufactured sliding screens work well for painting storage but cost can be a factor. Paintings can be stored economically on padded blocks at least six inches from floor. Interleave with acid-free cardboard. Place paintings back-to-back and front-to-front. Flat metal map storage can be used for unstable paintings. Hanging hardware should be removed before storage.

"Acceptable standards for temperature and relative humidity in a painting storage area are 65° - 75° F (18° -24° C) and 40% - 55% RH."xxix

Painting - shipping

Before shipping a painting careful assessment of the condition must be made. The painting assessment should be documented in a report and photographed. This report should accompany the painting and be checked by the receiver. Criteria to consider before loaning a painting:

-number of venues scheduled
-environmental fluctuations that will be encountered
-mode of transportation employed

General rules for restricting a paintings travel:

-recent treatment
-history of recurring problems
-recent losses
-unstable canvass: dry or brittle from age, likely to tear under stress; bulges indicating uneven tension of the canvas; loose canvasses.
-evidence of worm activity in a panel, stretcher, or frame
-delamination between layers of a painting that has been relined
-flaking varnish

Painting - frames

Frames should be given equal consideration during assessment. The frames main function is to protect the painting, aesthetic value is secondary. Damage includes:

-loose or missing ornaments
-active flaking of the paint or gild surface
-weak corner joints
-recent worm tunnels
-mold or fungus
-delamination of surface veneer

Frame rabbets should be padded to protect the edges of the painting with cork, felt, or velvet ribbon. No glass, paintings should be shipped with Plexiglas or UF-3 acrylic. Use spacers to keep Plexiglas away from painted surface.


XXVI Preble, D., Preble, S. & Frank, P. (1998) Artforms. New York, NY: Longman.
N.Matsukawa (2002). Natural resins and balsams. The Encyclopedia of painting materials. Author:
Accessed 2003 at,<>
XXVIII N.Matsukawa (2002). Natural resins and balsams. The Encyclopedia of painting materials. Author:
Accessed January 2003 at, <>
XXIX Moser, K. (1992) Painting storage: a basic guideline. In K. Bachmann's (ed.) Conservation Concerns: a guide for collectors and curators. Washington: Smithsonian Institution Press.