Long-term finish ageing and patina

Overview

No piece stays as it was the day it was made. Fine Woodworking puts it plainly: the surface gets a slightly rough feel, sunlight oxidises the surface cells, and hands leave oil and dirt. Some of that change is welcome and some is damage, and the two are worth telling apart. The sources used here are the USDA Wood Handbook (chapter 16) and a Popular Woodworking article by Bob Flexner. They are mostly about general principles; I found no source that quantifies ageing for particular species or finishes, so this entry gives none.

When to use

Read this when you are deciding whether to leave an aged surface alone, protect it, or refinish it; when a client asks why a table has changed colour; or when choosing a finish for a piece that will sit in a bright room.

Materials and tools

• Curtains, shades or a transparent UV-resistant window coating (Flexner)
• Tablecloths, placemats and coasters
• A humidity gauge, if you want to track the room
• The care routines in Cleaning and care of finished furniture

Steps

1. Know what is changing. The Wood Handbook says extractives, the chemicals that give heartwood its colour, change colour under UV or visible light, and the change may be lighter or darker. Some species change within minutes of outdoor exposure. Wood also changes colour indoors: ordinary window glass blocks most UV, so visible light causes most indoor colour change, and UV stabilisers in a finish do not necessarily prevent that.
2. Expect the finish to age too. Flexner says finishes lose their see-through, depth-giving qualities through oxygen, UV light and physical abuse. Oxidation is very slow, but UV accelerates it and can dull and craze a finish within a few years if exposure is severe. Flexner says nothing can slow oxidation of a finish and still leave it usable, but light and physical abuse can be reduced.
3. Reduce light. Place furniture away from windows, or cover windows with curtains or shades or a UV-resistant transparent coating. Flexner's photographs show a walnut chest back faded and crazed where a window hit it, and a drawer front crazed everywhere except behind the hardware, which shielded it.
4. Reduce abuse. Use tablecloths when a table is idle, coasters and placemats, and moving blankets when shifting furniture (Flexner).
5. Manage humidity if you can. Flexner says wood movement from changing relative humidity is the main cause of splits, warps and failed joints in indoor furniture. A constant humidity near 50 percent would prevent most of it but is practical only for museums; a finish in good condition slows moisture exchange so the season changes before the wood moves far. See Wood moisture and acclimation before finishing.
6. Decide when to refinish. Flexner's rule of thumb is that if the finish no longer does its job, refinish it, unless the piece is valuable or historically important enough to keep in museum-like conditions. For a valued antique, consult a conservator; this guide does not cover that.

Common problems

• Darkening or yellowing. Penetrating oils themselves can add warmth: Fine Woodworking's author notes that boiled linseed oil adds a little yellow and makes a piece look older. Whether this is patina or a defect is a matter of taste (Boiled and raw linseed oil, Oil and wax blends).
• Fading. Faded areas where objects sat show light's effect; the Wood Handbook says the change can be lighter or darker. Cleaning does not restore colour; the Handbook says removing mildew reveals original colour and does not add any, and that weathered wood looks silvery grey because weathering removes coloured components.
• Crazing and cracking. Flexner says thinner coatings are less likely to crack or craze, and very old furniture that survives well usually has very thin finishes. Thicker films (four or more coats) protect better but may age worse.
• Veneer or joint failure. Cross-grain movement and aged, brittle glue (Flexner). Not a finish problem, but a finish can slow it.
• Outdoor weathering. Much faster; see Outdoor finishing. The Handbook gives about 6 mm of wood lost to erosion per century as a general figure for exposed wood.

Safety and environment

Ageing is not a hazard, but old paint of unknown composition may contain lead; 16 CFR 1303 limits lead in new consumer paint because of the poisoning risk to children (see Toy and child-safe finishes). Sanding old finishes makes dust; wear a respirator suited to dust (Finishing shop ventilation and PPE). Do not assume an old finish is safe to disturb without knowing what it is. See Finishing safety and disposal.

Related

• Cleaning and care of finished furniture, Repairing finish damage, Stripping and refinishing
• Distressing and antiquing, Glaze and antiquing glaze: faking age on purpose
• Teak and oily wood outdoor care, Outdoor finishing
• Paste wax, Furniture polish and finish revivers