I would not use anything other than a food safe oil on a rattle that is going to be used. I would also avoid oil used for cooking as I mentioned elsewhere as some can go rancid and other especially like walnut oil because of containing nuts. Mineral oil is a good one to use. so unless it states on any finish that it is food safe then don't use it, this is my personal thoughts on this
Not aimed at you specifically, Derek: I think a bit of clarification on what “oil” means and its characteristics is important. There are a whole host of sources and types.
Petroleum oils are pumped from the ground and are broken down and refined into by-products like gasoline, solvents, waxes, and oils. Mineral oil is a commonly recommended petroleum by-product in woodworking. These types of oil will never cure and will remain “oily” to the touch. (I never put mineral oil on any wood item because of this.)
Plant oils come from a variety of sources but behave quite differently.
Most cooking/food oils (corn, olive, safflower, peanut) behave like mineral oil and will not cure. These may turn rancid when applied to wood, but I think you would need a very thick coating for that to happen. I do put cooking oil on spatulas and spoons but I’m sure the frequent exposure to boiling water or the soapy sink removes much of it.
Tree nut oils commonly seen are tung and walnut oils. These oils will naturally cure to a non-liquid state so don’t yield in an “oily” feel. We turners use and recommend them quite frequently as they’re easy to work with and are non-toxic when they don’t contain heavy-metal driers (make sure you are buying 100% pure oil). Tried & True products are in this category. As I understand it, the proteins which cause allergic reactions are removed in the refining process, so it’s not really a risk using them. As I recall when I was researching this concern, the US govt. has a few online resources citing the safety. I frequently use these oils for all of my food bowls and kitchenware.
Commercial finishing products like wipe-on varnishes and paints use a lot of tung and linseed oil in their formulations but heavy-metal driers are added to speed up the curing process. Heavy metals are bad for your long-term health outlook, but, once the finish has cured it becomes non-toxic. If I were to apply one of these finishes, I’d make sure to wait 30 days or so before I used it for food or baby toys.
If you’re curious whether an oil will cure, expose a small puddle to the air and let it sit for a few days. If it gets a wrinkly, skinned appearance it is curing. If the skin doesn’t appear then the oil will remain oily and will hold dirt and grime making the item look grungy and dirty — think kitchen range hood, especially a commercial kitchen range hood!
