Linseed oil is made from the seed of the flax plant. Available in many forms: Brown, boiled, raw and refined. It is used to coat various metal powders or filings you want to keep from oxidizing or reacting with other chemicals. The coating preserves the metal with a varnish coating. Adding some solvent will help distribute the oil and ensure complete coating. You must screen your metals while drying, otherwise it will turn into hard clumps. The cheapest form (usually boiled) is suitable for fireworks.
Linseed oil products are available at paint and hardware stores.
Rags dampened with boiled linseed oil are a fire hazard, because they provide a large surface area for oxidation of the oil. The oxidation is an exothermic reaction which thermal runaway accelerates as the rags get hotter. Such rags should be washed, soaked with water or incinerated to avoid unexpected spontaneous combustion.