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Preserving paintings and frames

Overview

Paintings are some of the most precious items in museum and gallery collections and should be treated with care and respect.

Whether it's a medieval altarpiece, impressionist masterwork or quirky modernist canvas, every painting requires a plan of care. This guide will examine how to look after easel paintings, that is, paintings which can be moved as opposed to wall paintings.

Equally important are the frames that hold these paintings. Many frames come with a unique history and design, further enhancing a museum's artistic collections.

Minimum equipment needed

Every member of staff in a gallery, from curators to volunteers, should have a good knowledge of caring for paintings and frames.

Many of the common problems associated with paintings and their frames can be identified with the human eye. Take a magnifying glass, torch, pen and paper around your galleries to monitor your collections with ease. Keep an ordered list of your paintings and their potential problems so you can bring specific issues to conservators.

Even small improvements can make a significant difference for the long-term well-being of paintings and frames in your collection.

Painting materials - Supports

Before painters can begin, they need something on which to paint. Methods for supporting paintings have changed throughout the centuries. We've outlined some of the most common materials and components of painting supports and explained how they might deteriorate over time.

Click on any unfamiliar words to find their definition in the guide's glossary.

Auxiliary or secondary supports

These are the terms used to describe the wooden structures to which a canvas is attached. The canvas or fabric is usually attached to a wooden stretcher or strainer using tacks or staples. This component of a painting is usually invisible but carries significant risks of deterioration.

Look out for:

  • Woodworm infestation, signified by frass
  • Splits in the wood cause by inappropriate display or handling
  • Warping caused by fluctuating relative humidity
  • Rusting or missing tacks
  • Dropped or missing stretcher keys

Fabric supports

Canvases have changed within the last two centuries - where once they were made of flax and 100% linen, the quality of painting fabrics became a lot more inconsistent from the mid-19th century onwards. Mechanised production methods and inferior fibres made canvases less durable. Since then, canvas lining has been introduced to help conserve poor-quality fabrics.

Fabric can deteriorate in unfavourable conditions. Fluctuating relative humidity and temperature can cause planar deformations, delamination between original and lining canvases and mould. Incorrectly mounted canvases can lead to splits in the tacking margins or canvas distortions if stretcher keys are dropped between the canvas and supports.

Although many painting fabrics are durable, poor handling can lead to punctures and tears.

Rigid primary supports

Before canvases, painters used wood panels as their primary support. 2nd-century CE Fayum portraits from ancient Egypt are among the earliest known works to use this method. In the 19th century, colourmen introduced millboard panels and other mass-produced rigid supports.

As new materials were developed, the 20th century saw the use by artists of hardboard, plywood and other reconstituted materials. Other rigid supports include glass, metal and ivory.

If your painting is on a rigid primary support, be aware of deterioration risks such as:

  • Movement along wooden panel joins
  • Splits within panels
  • Woodworm
  • Distortions caused by cradles
  • Weak or delaminating corners on manufactured boards

Painting materials - Paints

Since the earliest days of humanity, people have painted. Your collections could contain wall-sized canvases covered in thick oil paints or they could be elegant acrylic works from modern masters. All paintings are crafted from hygroscopic materials and all of them deteriorate in some way.

Each different layer of a painting poses some risk of deterioration. We've outlined the main materials in paintings and their associated issues.

Preparatory layers

Size

Size is an invisible solution used for stiffening fabrics and canvases, is applied to paintings to reduce the absorption of priming, paint and varnish into the fabric. This can deteriorate as the result of direct water damage or high relative humidity levels. Occasionally it could cause priming and paint to delaminate from the canvas.

Priming and ground layers

These serve two functions. They provide a suitable surface on which to apply the paint and create a tonal base for the composition. It is generally a stable layer, but if the artist used experimental materials then the paint could flake or tent.

Paint layers

Paint is, in its simplest form, the mixture of pigment and medium. The medium varies according to the type of paint. Oil paint binds using linseed oil, watercolour uses gum arabic and acrylic uses a synthetic resin.

The stability of a paint layer depends as much on its method of application and/ or adulteration with additives (a popular 19th century development), as it does on environmental considerations. Many conservators specialise in the deterioration of paint and are able to identify problems based on the shape and type of cracking. Consult one of these experts if you are unsure of the issues facing your paintings.

Below are some of the terms, explained in the glossary, they may use to describe deterioration in paint:

  • Flaking
  • Stretcher marks
  • Cupping
  • Tenting
  • Blistering
  • Cleavage
  • Drying cracks
  • Alligator cracks
  • Craquelure

Varnish layers

Most paintings will be finished with some kind of varnish to give it an extra depth, subtlety and protection. Historically, varnishes were made using natural resins such as dammar and dusk, but in the 20th century use of synthetic resin varnishes became commonplace.

Varnishes are usually clear upon application but can discolour with age. Natural resin varnish will yellow significantly, altering the tonal values of paintings and distorting the appearance of colours. From the late 19th century onwards, some artists preferred to leave paint surfaces unvarnished. Look out for blooming, blanching and degradation, especially if light intensity levels are high.

Framing materials and deterioration

Frames are more important than many people will initially consider. An original frame is an integral and historical part of a work of art. It isolates the painting visually, separating it from its surroundings with a decorative design that enhances and protects the work.

A history of frames

Traditionally, the making of a frame was a partnership between joiner, carver and gilder. Until the mid-16th century, frames were engaged, which meant that they were built into the structure of the wooden primary supports.

Frames used to be carved from wood but the advent of composition, a mixture used for moulding ornamental shapes, replaced carving as a significantly cheaper alternative for elaborate designs. As a result, frames became more increasingly elaborate until the 20th century when polished or stained wood came into vogue.

This diversity of framing materials mean that galleries could contain a wide array of frame types. We've outlined some of the materials that you might find on frames below.

Substrate or base materials

Composition, wood and papier-mâché are the predominant materials for creating a substrate, or base for a frame. Most problems with frames emerge from deterioration of the substrate.

Be aware of the following risks with base materials:

  • Open mitre joints
  • Splits in the frame, cause by either bad handling or fluctuating relative humidity
  • Woodworm infestations, indicated by frass

Gesso

Gesso is a mixture of whiting and size applied to the substrate to create a smooth surface area for gold-leaf gilding. Often, several coats of gesso will be applied, with a slightly thinner consistency for each layer. Once dry, gesso takes on a plaster-like appearance, then it is smoothed until the frame has a smooth enough surface for gilding.

Gesso, if exposed to fluctuating relative humidity, can begin to flake from the frame and take gilding with it.

Bole

Bole is a mixture of fine clay and size applied on top of gesso and contributes to the final appearance of the gilding. Yellow bole is usually associated with oil gilding, and yellow, red and black bole with water gilding. Bole itself is stable, but it can flake or raise if it is applied to unstable gesso.

Gilding

Gilding is the process of applying gold leaf to give frames a shimmering, ornate finish. Two techniques, water gilding and oil gilding, result in a slightly different appearance. Both require gold size to act as an adhesive for the gold leaf.

The differences between the two are subtle. Water gilding is burnished and placed over coloured boles, whereas oil gilding is often left matt and used on composition frames.

Gilding can flake if the relative humidity is unstable. It is more likely to be damaged by bad handling, where it can receive scratches and abrasion.

Toning layers

This delicate final layer is a slightly pigmented glaze that can be applied over gold leaf. Toning layers can be damaged very easily. Look out for patchy surfaces, fine curling and flaking leaf as the result of poor handling.

Toning layers should not be confused with coatings, added many years later in an attempt to revive old frames. Coating should be avoided as it can often damage the gilding.

Environmental threats

It may seem, at times, like everything in a museum is out to damage or deteriorate your paintings. Paint can suffer from just about every environmental threat, while the gesso and gilding in frames are particularly sensitive to changing relative humidity. However, with a bit of care and attention you can put in simple precautions to protect your museum's invaluable items.

Light

Lighting your museums and galleries correctly is an essential part of collections care. Materials used in painting vary in their sensitivities and reactions to light. For example some pigments are stable, or lightfast, while others fade even at low light levels.

Light damage is both cumulative and irreversible. It takes only decades for the most sensitive colours to fade, so keep light levels as low as tolerable in order to slow the rate of fading. High levels lead to rapid colour loss. Frames generally remain stable in light levels that would be considered damaging to paintings.

Lighting guidelines

  • Aim to keep light levels for painting at or below the recommended light intensity of 200 lux.
  • When this isn't achievable, reduce the amount of time that a painting is exposed to counter cumulative damage.
  • Apply UV-absorbing film to windows to reduce the damage done by the ultraviolet in daylight.
  • Control daylight with window blinds.
  • When artificial lighting is used, fit low-UV emitting tubes.
  • Use track lighting, which produce cooler and more diffuse light than spotlights.
  • Do not expose paintings to unfiltered UV-emitting fluorescent lamps.
  • Do not use individual picture lights as they produce 'hot spots' on the surfaces of paintings and frames.
  • If using incandescent lights, take measures to reduce the amount of heat emitted.

Relative humidity

Relative humidity (RH) is the way we measure how 'wet' or 'dry' the air is. Monitoring and controlling relative humidity is essential to preserving both paintings and frames.

When RH fluctuates, it causes the expansion and contraction of many materials in the painting. Wood, gesso and fabric readily absorb moisture in the air, which causes this growing and shrinking, ultimately weakening the materials.

Paint can crack or flake as a result of the expansion and contraction of the supporting materials. Canvas can become slack and sag, frame joints and wooden panels can open or split, while the gilding can deteriorate quickly.

Stick to these basic guidelines to prevent this often irreparable damage:

  • Aim for RH between 50 and 60% for both stores and display areas.
  • If necessary use humidification or dehumidification equipment.
  • Mould typically grows at at an RH of 65% or above.
  • Low RH levels causes paintings to dry out and become brittle, making them susceptible to damage.
  • Low RH can also damage frames by causing the joints to dry out and open up.

Temperature

Temperature can affect paintings and their frames by changing the RH and damaging some of the materials.

We recommend following these guidelines for controlling temperature in a museum:

  • Stick to a 'human comfort' temperature of around 20ºC - sudden fluctuations can be damaging.
  • Keep any fluctuations in temperature gentle and to a differential of no greater than 5ºC.
  • Adjust conditions using low-level conservation heating.
  • Use blinds to reduce heat gain and loss.
  • Do not hang paintings above heat sources as the rising hot air carries dirt.
  • Sub-zero temperatures can cause acrylic paintings to grow brittle and crack.
  • High ambient temperatures can soften paint and other constituent materials, which then causes dirt to stick and become absorbed into the work.
  • Excessive heat dries paintings and speeds up natural ageing.

Air quality

Pollutants in the form of gases and particles can cause or accelerate deterioration in paintings, as chemicals react and cause discolouration and fading.

Gaseous pollutants are not a pressing problem for paintings and frames, although be aware that impure gold leaf is more likely to be tarnished than pure gilding. High visitor numbers and poor housekeeping can lead to an accumulation of dust, which is more dangerous to paintings and frames. The mixture of particles attracts moisture and can become absorbed into the artworks.

Remember these crucial points when protecting your paintings from pollution:

  • Good housekeeping is the best prevention of dust pollutants.
  • Door seals and dust-trap mats stop a large amount of external pollutants.
  • Fit glass in frames to protect particularly vulnerable, unvarnished paintings.
  • When cleaning glass in frames, hold a piece of card against the gilded edges to protect from abrasion.
  • Never clean the surface of a painting, even to remove dust as it can remove paint and varnish. Seek expert advice.
  • Do not dust frames, particularly with cloths, as dust contains abrasive particles and will abrade fragile gilded surfaces.
  • Oil gilding is damaged by organic solvents such as turpentine and white spirits.
  • Water gilding is easily damaged by moisture.
  • Unvarnished paint surfaces, especially acrylic paint, attract and absorb dust, becoming engrained, sometimes irreversibly.

Biological threats

Woodworm

Wood is everywhere in paintings - in the stretchers, strainers, frames and panel supports. All of this wood is susceptible to woodworm, which you will notice by the presence of frass, a fine, sawdust-like debris.

When tackling woodworm, remember:

  • Adult beetles are usually active in late spring and early summer.
  • The structure of wood is compromised by females laying eggs in the wood, which turns into burrowing larvae which feed on the wood.
  • The holes in wood are made from adults emerging after two years of developing in the wood.

Mould

Mould readily develops on paintings and frames in conditions where the relative humidity is above 65%. You'll find it on the backs of canvases, attracted to dust and the size coating. Pay particular attention to glazed paintings, as microclimates can form between the paint and glass or inside the glass itself.

Minimising biological threats

  • Isolate a painting or frame with active woodworm, wrap it with museum-standard materials and remove it to a quarantine area for further assessment by conservators.
  • Follow the same procedures for items affected by mould, except keep them in a well-ventilated space.
  • Keep RH levels below 65% to prevent the outbreak of mould.
  • Always contact a conservator to treat paintings or frames with signs of mould and woodworm.
  • Insects can leave very acidic material including excrement called 'fly specks'.
  • Wear mouth and nose protection when dealing with mould to avoid ingesting spores.

Storage

One of the biggest threats to the conservation of paintings and frames are humans themselves. However, human interaction is necessary for the storage, display and appreciation of paintings. Take simple precautions to eliminate human damage to your collections.

Storage guidelines

  • Always store paintings and frames vertically unless a paint layer is unstable, in which case store horizontally and contact a conservator.
  • For sliding rack systems, attach 'S' hooks, not stretchers and strainers to framed paintings.
  • Ensure pigeon hole-style storage is not overfilled.
  • Store paintings according to size.
  • Put a rigid interleaf of chemically stable materials between paintings.
  • Do not store framed and unframed paintings in the same section.
  • Remove protruding hooks, nails and chains if necessary.
  • If paintings have to be stored on the floor against a wall, utilise high-density foam blocks to raise them at least 20cm off the floor.
  • Stack in descending order of size.
  • Stack paintings front to front and back to back with interleafs in between.
  • Use appropriate wrapping to cushion the corners of vulnerable frames.
  • Avoid stacking under windows or near doors and radiators.

Handling

The principal cause of human damage to paintings and frames is through unsafe handling practices. Museum life necessitates handling, whether that's during installation, moving things to storage or rearranging exhibitions. Anyone handling paintings and frames should be trained in the appropriate procedures.

Simple common sense can go a long way when handling paintings and frames. Plan ahead, take your time and clear your walking routes and destination of obstacles. You need to decide in advance if the painting is secure enough to be moved - check that the painting is secure in its fitting and that no stretcher keys are loose.

Handling considerations

  • Small paintings can be handled by one person, but only carry one painting at a time.
  • Have clean and dry hands or wear gloves if you can get a good enough grip.
  • Hold one bottom edge and one vertical edge, never carry a painting by the top of a frame.
  • Never touch the surgace of a painting or the back of its canvas or panel.
  • Carrying larger paintings requires more than one person with one clear leader. Use the blocks on the back of the frames, it's why they are there.
  • Use webbing straps or trolleys if appropriate.
  • Carry flaking paintings horizontally and move to a flat storage space. Contact a conservator to secure the paint.
  • If a painting has loose mould, secure the section with acid-free tissue before moving.
  • Never let tape come into contact with gilded or decorative surfaces.
  • If damage does occur, gather the pieces, no matter how small, and identify where they came from.

Packing

The underlying principles in wrapping paintings is to protect any vulnerable surfaces from puncturing and make them heat, cold and moisture resistant. When paintings and frames go into storage, they should be preserved against changes in RH and temperature.

Always keep stocks of museum-standard soft packing materials, such as:

  • Bubblewrap®
  • Jiffyfoam®
  • Acid-free tissue
  • Polyester film (12 micron Melinex®)
  • Polythene
  • Low tack tape
  • Parcel tape
  • Cotton tape

Packing guidelines

  • Use an interleaf such as acid-free tissue or Melinex® around the gilding of frames and over the paint surface of unframed paintings.
  • Use Bubblewrap® or Jiffyfoam® to make protective corners.
  • Make temporary backboards if a painting does not have one and cover the front of a painting with board during transport.
  • Protect glass with low-tack tape.
  • Attach new labels to backboards and or the backs of frames.
  • Label packages on the back and front.
  • Always retain existing labels, even fragments, for the information they contain.
  • Don't use Bubblewrap® directly against paint surfaces.
  • Don't let any adhesives touch gilded surfaces.
  • Never stick labels directly onto the back of canvases or panels
  • Don't try and remove labels from the back of canvases or panels.

Transport

Ideally, all paintings would be moved in climate-controlled, 'air ride' vehicles, but prohibitive costs and lack of trained staff make that unlikely.

However, there museums can still take basic measures for good practice when employing carriers:

  • Ask around for recommended carriers, conservators and curators who know the job and have experience in transporting artworks.
  • Look for carriers with clean and well-equipped vehicles, ensuring that there is plenty of padding and no loose items.
  • Build up a relationship with carriers so that they understand the museum's specific requirements.
  • Explain why your requirements are so meticulous.
  • Ensure paintings are secured to the side of vans with flat-webbing and additional padding and raised from the ground to absorb vehicle vibrations.
  • Do not allow paintings to travel if they are inadequately packed.
  • Make sure vans are heated if the weather is cold. Sub-zero temperatures can cause damage to acrylic paints.

Tips for conservation

Correct framing can contribute significantly to the preservation of a painting. Ideally, this would mean enclosing all paintings in rigid, well-sealed frames but this is not a realistic option.

Any painting that is poorly fitted, not secure in its frame or unprotected at the back is vulnerable to damage. However, intervention techniques can impact the frame, even if it is simply a straightforward refitting. Simple procedures such as attaching metal brackets carry risks - if the screws are too long or the holes too deep, frames can be damaged.

As such, we recommend that refittings are carried out by conservators who understand the unique needs of different materials and techniques.

Refitting specifications

  • Rebates should be padded to prevent friction on the face of the painting.
  • Spaces around the sides of the painting should be packed with suitable material to avoid movement of the painting within the frame.
  • Metal brackets or plates should only be used to secure paintings within frames - hammers, nails and staplers can cause dangerous vibrations.
  • Plates should never be secured directly to the painting, keeping the paintings held in by pressure alone with provision of the expansion or contraction of the support. Think more carefully about plates on multi-panel artwork.
  • Fit backboards in suitable materials such as Correx®, Fome-cor® or hardboard to protect against damage to the reverse of a painting. This also keeps out dirt, buffers against vibrations, shock and RH levels.
  • Adapt frames if necessary to secure backboards.

Considerations for fitting glass

  • Is the frame sound and strong enough to support the weight of glass?
  • If not, can the frame be adapted to take extra weight?
  • Consider acrylic cover if glass is too heavy.
  • Space needs to be left between the glass and the surface of the painting, using checksticks or spacers.
  • Decide on a thickness of glass suitable for the frame.
  • Laminated or toughened glass might provide extra protection.
  • UV-filtered and low-reflecting glass can protect against light damage.

Ask a conservator

With any issues related to preserving and protecting frames, if you are in any doubt consult an expert conservator. Accredited conservators have years of experience in their particular fields, so seek advice to learn from their wisdom.

Resist the temptation to carry out basic procedures yourself as even simple treatment can result in unforeseen damage. Something as simple as tightening a slack canvas can result in tearing, over-expansion or splitting tacking margins when executed incorrectly.

Further advice

Read more advice guides from Museums Galleries Scotland and the Collections Trust. Contact us with any further questions.

Useful books

  • The Care of Pictures (Stout, G.L. Dover Publications Inc, 1975, ISBN 0 4 86 23165 8) An earlier edition (1948) is available online.
  • The Artist’s Handbook of Methods and Materials (Mayer, R. Viking Press, 5th edition, 1991, ISBN 0 6 70 83701 6)
  • Conservation of Paintings (A Close Look series Pocket Guide) (Bomford, D. National Gallery Publications Ltd, 2009, ISBN 1 8 57 09441 7)
  • Learn to Frame (Fairbrass, S. William Collins and Sons Ltd, 1990, ISBN 0 00 412402 2)

Glossary

Acrylic - Resin is the medium for modern synthetic paint, used by artists since the 1950s.

Alligator cracks - A pattern of drying cracks that resemble alligator skin and are associated with different drying rates of paint layers.

Blanch(ing) - An opaque whiteish discolouration on the surface of a painting, occurring in the binding medium or within the varnish film itself.

Blistering - A convex deformation of ground, paint and/or varnish.

Bloom - Areas of blueish cloudiness in the varnish layer typically caused by contact with moisture, unlike blanching it only affects the varnish layer.

Bole - A fine clay mixed with size, used on frames to prepare a smooth surface for water gilding; applied on top of gesso and available in a wide variety of colours.

Buckling - Rigid bulging distortions in the canvas support, often at the corners.

Burnishing - The process of polishing water gilding with an agate burnisher.

Cleavage - Separation between layers of paint, paint and ground, or ground and support.

Cockling - Rigid rippling distortions across or at the sides of the canvas support.

Colourmen - Suppliers and manufacturers of artists’ materials, e.g. Winsor & Newton.

Composition - A pliable mixture usually made of whiting, glue, resin and linseed oil from which moulded ornaments can be made, also known as compo.

Cradle - A grid of wooden bars running horizontally and vertically across the back of a panel painting fitted with the aim of preventing warping without restricting movement.

Craquelure - A network of random cracks over the surface of the painting, caused by drying, ageing or changes in RH.

Cupping - Islands of aged paint, separated by cracks, with upward curving edges like shallow cups.

Degradation - Used to describe the breaking down of a varnish layer so that it becomes opaque, typically in patches.

Dammar - A vegetable resin derived from trees that can be made into varnish.

Delamination - The separation of layers, e.g. of paint layers, ground or varnish layers, and also used to describe separation between original and lined canvases.

Drying cracks - Occurs in the paint or vanish films during the drying process; typically these have rounded or sloping edges and no cleavage.

Engaged frame - A frame made from the same piece of wood as the panel, or which was attached when the panel was constructed.

Flaking - An unstable condition in which particles of paint, or paint and ground become detached.

Frass - Fine sawdust-like debris produced by wood-boring insects, e.g. woodworm.

Gesso - A mixture of whiting and size used as ground for gilding.

Gilding - The application of gold leaf to a prepared surface.

Glaze - A transparent layer of paint, usually associated with oil painting.

Gold size - The adhesive used to fix gold leaf in place.

Ground - An opaque white or coloured coating applied to the support, the materials vary.

Gum Arabic - Water-soluble gum obtained from the acacia tree, with a variety of uses including as a binding medium with pigment for water-colour.

Hygroscopic - Describes a substance which readily absorbs moisture from the air.

Lining - The process of adhering a new fabric to the back of a painting on canvas.

Loose lining - A fabric stretched directly behind, but not adhered to, a painting on canvas.

Mastic - A vegetable resin derived from a tree and used as a varnish.

Medium - The film-forming material holding pigment particles together, e.g. a drying oil such as linseed, in oil paint.

Millboard - Manufactured alternative to wooden panels introduced in the late 18th century, made of mill and paper waste; tend to be stable though corners soften.

Mitre joint - A diagonal joint used at the corner of frames with the adjacent sides abutting.

Moulding - A shaped projecting or recessed band running along a frame which may be plain or bear carved or moulded ornaments.

Oil gilding - Using an oil-based gold size as adhesive and was the standard process for picture frames in the 17th century; unlike water gilding it is usually left matt.

Papier-mâché - Material made of pulped paper soaked in a binder and used as a moulded ornament for frames.

Pigment - Coloured particles in powder form that become paint when combined with a medium.

Planar deformations - Describes surface distortions of the support.

Priming - Now often used synonymously with ground, historically used to describe the layer following the ground providing modified colour or texture on which to paint.

Rebate - Recess beneath the sight edge of a frame designed to take the picture.

Relative Humidity (RH) - The measurement of the amount of water vapour present in the air.

Resin - Natural resins are secreted or excreted by certain plants; synthetic resins are produced by chemical synthesis and are widely used in conservation.

Size (re: paintings) - When used for paintings, is a solution or gel applied to seal raw canvas and traditionally made from rabbit-skin glue or gelatine; for frames, an adhesive used to make gesso and also used to protect unburnished water gilding, traditionally made from animal skins or parchment clippings.

Strainer - A wooden frame with fixed, non-expandable corners over which a canvas is stretched.

Stretcher - A wooden frame with expandable corners over which a canvas is stretched.

Stretcher keys - Small wooden wedges that fit into slots on the inner corner joints and in cross-bars of the stretcher; they are used to adjust the size of the stretcher.

Stretcher marks - A line of cracks or deformation in the painting surface that follows the line of the inside edges of the stretcher or strainer.

Tacking margins - The part of a canvas that wraps around the edges of the stretcher or strainer.

Tempera - An aqueous binding medium, traditionally used to describe paint made of pigment mixed with egg as the medium.

Tenting - A type of cleavage where the paint, or paint and ground, layers are cracked and are forced upward in a tent shape.

Unlined - Describes a canvas which has not been through the lining process.

Varnish - A clear solution of resin dissolved in oil or solvent which dries to form a transparent film.

Water gilding - Gilding using a water-based gold size as adhesive which came into fashion at the end of the 17th century and can be burnished to a high sheen.

Whiting - An essential component of gesso and composition, made from natural chalk (calcium carbonate) in Northern Europe or gypsum (calcium sulphate) in Italy.