In the dictionary varnish is defined as "resin dissolved in a liquid for applying on wood, metal, or other materials to form a hard, clear, shiny surface when dry". This definition however is a fairly narrow and industrial view of how varnishes are used.
Simply put, a varnish is a final protective coating that is applied to a painting or artwork once finished. Varnish protects the artwork from environmental dirt and damage, and in some cases it serves as protection from sun damage as well.
When it comes to fine art paintings, varnishes are traditionally used by oil and acrylic artists, although modern mixed media artists and watercolourists may also employ a varnish. Varnishes for fine art purposes are generally some sort of resin (natural or synthetic) dissolved in water or a solvent which will then dry, cure and harden, providing long-lasting layer of protection. Fine art varnishes should also be removable in case the artwork is ever taken to a conservator where the dirty/damaged varnish layer will need to be safely lifted.
Types of Varnish
When the time comes to varnish, there are different kinds of varnish to consider, each with their own advantages and challenges.
Oil Varnishes
Oil varnish is traditionally made from Damar resin; tree sap from Asian conifer trees from the Shorea family. This particular resin is chosen for its clarity, it's nearly colourless/pale yellowish hue and for it's hardness. To make varnish damar resin crystals are dissolved in a strong solvent (turpentine, specifically) and the resulting sticky solution is a varnish. This thick and glossy varnish is often referred to as Picture Varnish.
Picture varnish is available in Satin and Matte from some brands, however the natural gloss sheen is reduced by adding a particulate to the solution. This particulate additive will also unfortunately dull the vibrancy of the colours and may result in a foggy or hazy appearance on the painting and for these reasons Satin and Matte oil varnishes are less common.
A very dilute damar solution creates a Retouch Varnish; a re-workable mid-process varnish that is intended to be painted over. An application of retouch varnish will even-out the surface sheen of an oil painting so the artist can see what areas are dead (sunk-in) and need retouching.
Using Oil Varnishes
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Acrylic Varnishes
Acrylic varnishes are made from acrylic polymers suspended in water. When the water has evaporated and the varnish is cured what's left is a durable acrylic film.
A polymer is just a fancy word for a really long molecule so an acrylic polymer is just a long chain of plastic molecules. Acrylic is a broad name for countless numbers of plastics that can by polymerized to create different working consistencies (thick, thin, grainy, watery, hard, soft, etc...) Polymers used in acrylic paints can be thick and heavy or soft and fluid or anything in between, however polymers used for varnishes are intended to be both hard and flexible when dry and have a high degree of clarity.
The disadvantages to acrylic varnishes
- Only suitable for use over water-based paints; cannot be used over anything oil-based
- Can bond permanently to acrylic paints making varnish removal difficult
- Adding an isolation coat before using an acrylic varnish is highly recommended
Hybrid Varnishes
Aerosol Varnishes
