If your soft pastels get grubby in a box mixed up together it is an easy task to tidy them up again.
Using rice flour (also called ground rice or powdered rice) you can roll them around and the dust accumulated from the other colours in your box will stick to the rice flour and your pastels will be nearly back to their former glory. They will be clean enough that they work just fine.
Here are some before and after pictures:
The rice flour most pastel artists use is easy to find in the dessert or baby food section of the supermarket. It is very reasonably priced. You can put some in a dish and roll the pastels around, you can put some in a bag and shake the pastels around or you can keep some in the bottom of your pastels box and let your pastels tidy themselves up over time.
Your pastels will work fine even if they are grubby.
Here is a picture showing a light coloured pastel covered in dark dust but it makes a light mark straight away.
When using them while they are grubby the only real trouble is telling what colour your pastel will be if they all look dark. If you look at the border of many pastel artists’ work you will see lots of little marks where they were checking not only the colour of a mucky pastel but how it looks against the colour of the paper they are using.
Here is a pastel painting I did using a mixture of tidied up and mucky pastels from the top grubby box. I used seven colours of Sennelier and Unison pastels: two reds, two greens, a blue, a brown and a nearly white on Tim Fisher 400 paper in colour Gold. Read more about this new paper in this earlier post.
The Soft Pastels Department on the Jackson’s Art website.