Walnut oil and other food oils

Overview

Food oils fall into two groups, and the difference decides what happens on the wood. Drying oils such as walnut, linseed, tung and hemp react with air and harden. Nondrying oils such as olive, corn, peanut and safflower stay liquid. The USDA Wood Handbook says the nondrying vegetable oils penetrate wood but do not cure, are food for microorganisms, and can become rancid. Wikipedia says walnut oil is a drying oil with no yellow tint and that some woodworkers favour it for food-contact items such as bowls; it adds that rancidity is not an issue because walnut oil dries when applied to wood in a thin coating.

Pros

• Familiar, food-derived ingredients; no solvent smell.
• Walnut oil dries without much yellowing and can be wiped on easily.
• Easy to renew; repairs blend in.
• Sold in culinary form at supermarkets, and as wood-specific products.

Cons

• Nondrying oils (olive, corn, peanut, safflower) can go rancid; the Wood Handbook warns they feed microorganisms.
• Drying oils need patience; the Wood Handbook says drying oils on boards may need several weeks to cure.
• Light protection only; cutting boards need regular re-oiling.
• Culinary walnut oil is a tree-nut product: where anyone using the item may be nut-allergic, choose another oil. No source was found that quantifies the residual risk from a cured film.
• "Food safe" labelling varies by brand and is not a standard.

Suitability

Best on hard, close-grained woods used for kitchenware: Hard maple, European beech, European walnut, Black walnut, Black cherry. Not a durable finish for tables or furniture. Skip it on very oily woods such as Teak; test before use. For boards, compare Mineral oil for cutting boards.

Coats and application

Sand to 180 to 220 (Sanding schedule). For a drying oil, flood the surface, wait 15 to 30 minutes, wipe off all excess, and wait days between coats; thin coats cure and thick coats stay tacky. A walnut-oil and beeswax paste is a common homemade variant (Wikipedia gives about one third oil to two thirds beeswax). Example for a bowl: three thin coats of walnut oil over a week, then buff with a dry cloth, then a wax top coat (Beeswax and carnauba). For nondrying oils, apply liberally, let it soak, and wipe off. The Wood Handbook says the simplest board finish is melted paraffin (canning wax). RMP Cutting Board Oil is a non-drying blend; it can go over fully cured tung or hemp oil, but RMP says not to put tung or hemp over it.

Compatibility

Dry-oil coats can be waxed. A nondrying oil under any hardening finish will interfere with it. Do not mix drying and non-drying oils in layers without checking the maker's rules (RMP's instruction above). Film finishes perform poorly on cutting boards (Wood Handbook). See also Tung oil and Polymerized linseed oil (stand oil).

Care and repair

Re-oil boards when they look dry; wash with mild soap, dry thoroughly. Sand lightly to fix knife scars before oiling. For nondrying oils, plan on frequent re-treatment. For a finish gone sticky, wipe with solvent and allow thin recoating.

Safety and environment

Rags soaked with drying oils (walnut, linseed, tung, hemp) can ignite spontaneously. The USDA Wood Handbook says to immerse them in water and seal them in a water-filled airtight metal container. See Oily rag safety.

Food contact needs careful wording. The ingredients are foods, and 21 CFR 175.300 lists walnut oil, linseed oil, tung (chinawood) oil and several vegetable oils among drying oils permitted as ingredients in resinous and polymeric food-contact coatings; that is a list of permitted ingredients, not approval of your bowl. RMP calls its Cutting Board Oil and Hemp Oil food-safe or food-contact safe, and Tried & True claims FDA 175.300 compliance; those are manufacturer statements. The Wood Handbook says to be sure any finish on food items is safe, non-toxic and recommended for food use, and to check the label, the manufacturer, FDA or the extension service. See Food-safe finishes.

Related

• Mineral oil for cutting boards: the petroleum alternative.
• Beeswax and carnauba: waxes for board and bowl topcoats.
• Oil and wax blends: ready-made blends.
• Tung oil: harder drying oil.
• Oily rag safety: fire hazard.