Restoring Oil Varnishes

MPF Conservation is a full-service company specializing in conservation, restoration and preservation of furnishings of upholstered and non-upholstered objects, textiles and interior architectural elements in the Western United States; this page discusses oil varnishes, tree resin varnishes, and polymerizing oils.

Polymerizing oils were the primary source of varnish during the Renaissance. Those varnishes, unlike the varnishes of today, were cooked at high temperatures melting the resins. These were typically bulky and transparent, using the inherent physical principles of self-catalyzation to make the varnish hard. Oil-based varnishes applied by brush often were combinations of tree resins and oils, which, once cooked, provided long-lasting protection to wooden surfaces.

Unlike historic cooked oil varnishes, modern oil varnishes primarily cure after exposure to light. A self-heating process is activated, eventually causing the oil to harden to a reliable transparent film. This makes the modern vanishes easier to apply by cloth as well as brush, and quite controllable during application processes.

Many Mid-Century Modern wooden furnishings are coated with a variety of wipe-on oil-based varnishes which harden into a satin transparent sheen protective film allowing the grain and color of the wood to be the star. These films are sometimes infused with resins, waxes and even honey, but they should not be confused with the historically cooked, bulky oil varnishes used from the twelfth-sixteenth centuries.

Tung Oil Varnish

Tung oil varnish (also sometimes called “China Wood oil”) is an oil which should be in the genre of varnishes. It can be applied by rag or brush, producing a polymerizing coating which is transparent and bright. Multiple coats produces highly refractive glossy finish, and the gloss factor can be refined by the number of coats applied and/or rubbing with abrasives after curing. It has been used for both furniture, marine and architectural purposes; ancient boat builders applied it to the interior and exterior hull to waterproof the wood.

Tung oil is obtained by a cold process rather than cooking to obtain the oil. Extraction of oils by pressing seeds and nuts of the native southern Chinese tree species Vernicia fordi (above right). After extraction the oil can then be subjected to heating to ensure the oil remains stable on wood surfaces, and remains homogeneous and bright.

Tung oil mixes well with other oils and tree resins with the use of turpentine to create varnishes with other attributes; even paint and wax makers use tung oil for the glossy attributes as well as elasticity.

Tree Resin Varnish.

Tree resins varnishes are made with tree resins, such as sandrac, dammar, copal, benzoine and mastic which are suspended in turpentine, an oily spirit from the balsam family. This allows the tree resins to dissolve into solution without cooking.

Sandrac, dammar and mastic, above; below, Mitchell creating the tree resin varnish.

We make many of our own tree resin varnishes in a cold process as shown above, where Mitchell is suspending the resins. They were wrapped in cheesecloth then immersed in turpentine to dissolve the resins, creating a viscous coating.

This oil-based tree resin varnish was an excellent match for use with the badly damaged painted Imperial Furniture from the Crater Lake Science and Learning Center‘s collection. We used this mixture because this varnish was compatible with the historic painted finish.

The historic oil paint was beginning to flake, largely due to a novice restorer having coated the historic paint with pigmented mineral oils. The damaging oil wicked under the degrading historic paint, causing the paint to scale from the surface.

Thankfully we caught this mistake shortly after it occurred. After wiping the residual offending mineral oil off the surface we deep cleaned the painted surface by neutralizing the oils, using fine pumices.

We then were able to proceed with efforts to secure the damaged historic paint to its ground by applying our in-house tree resin varnish. The clear film which acted like a fixative and secured the paint for many decades, a success!

Because of the saturating nature of the viscous medium the furniture has a dark and somewhat glossy appearance, which normally would be mitigated with rottenstone. This gave the Imperial line its original appearance of dusty “old wood.” As we were brought in last minute with a deadline for the Superintendent’s Residence opening to become the Learning Center, all furnishings were installed without the final step of dusting with rottenstone. We hope to be able to finish the project onsite for the NPS in future.

Eames Chair circa 1960

In keeping with the original finish, we restored the finish using a Tried and True, an oil varnish whose base was linseed oil infused with beeswax and honey, and added an earth pigment to bring it close to the original color. As seen in Mid-Century Modern Upholstered Objects, the Brazilian rosewood early Eames chair shell was restored with a commercial varnish oil created by Joe Robson (image shown right, stolen from the Tried and True website) for his original Tried and True varnish line (which was purchased).

From their website, “Why use Tried & True? It is 100% Solvent free, Zero VOC’s, safe for food contact, made from all natural ingredients, and has superior coverage rate– up to 1000 square feet per gallon!”

Above, the exterior after treatment and after restoration.
Below, before treatment, left, and after treatment, right.

Coming soon, the use of both oil-based Tree Resin Varnish waxes and Shellac Varnish on the CCC Table and CCC Bench from Crater Lake National Park.

For discussion on other types of finishes, visit the pages below: