Oil pastels are great fun to play with, and they’re the perfect companion to have if you want to paint and sketch on the go. But there is one problem. They don’t ever fully dry. This is because oil pastels are actually made with mineral oil, and this is very different than the linseed oil which is used in traditional oil pastels. In addition to the mineral oil, oil pastels also contain wax which, as we all know, as not something that ever fully dries either.
So to answer the question. Oil pastels will never fully dry out on their own. They may get a bit of a protective layer on them over the years. But you’ll still be able to scratch oil pastels off with your fingernail even decades after finishing a piece.
But. There’s a solution to this! A few different companies have made some products which can be sprayed onto an oil pastel piece once it is finished, and it will seal the surface. The best one on the market comes from Sennelier and is available via the link below. Another option is to simply use a fixative on top of it. These are available via Krylon and other brands.
One thing to watch out for when using these sprays, is that if you spray an excess amount of these products onto your drawing it will cause them to run. This may seem cool, if you’re going for that effect, but if you’re not, then you just ruined your work. It’s best to spray your piece about 12 inches away with a fine mist, then wait a few minutes, and do another fine layer. You don’t want the spray to pool up anywhere on the drawing. If spraying your painting after you’ve finished your work isn’t your thing. Then maybe oil bars will be a better fit. Oil bars are made by a number of different manufacturers , there’s some from Winsor and Newton, as well as Sennelier. The benefit of using these bars, is that basically they’ve got a waxy outer coating on them, and are filled with a semi hardened form of oil paint. So you can paint with these, and blend with them, and it’s basically like using an oil pastel, but one that dries to the consistency of oil paint. They are a bit “wetter” so you can’t really work the surface with multiple colors as you can with the traditional oil pastels, but personally I just love Oil Bars. I use both in my work.
An alternative to oil pastels is pigment sticks or oil bars. Both are traditionally pigmented materials containing wax and linseed oil instead of non-drying mineral oil. Due to the wax, they never become truly solid, but an artist who likes the feel of oil pastel—yet wants the finished artwork to dry to a fairly high degree of hardness—may find pigment sticks or oil bars a satisfying alternative. This medium can be varnished with a number of traditional resin varnishes made for oil paintings.