Make your own pastels from saved pastel dust as it falls on the easel and/or combine with pastel ground you can purchase .. it's as close as you can get to PURE PIGMENT.
Here's the link to the instructions and supplies: https://userfiles.faso.us/94112/18270.pdf
TravelingArtista.com https://www.travelingartista.com/
DISCLAIMER: I am not a chemist nor professional pastel maker, but sharing my personal experiences to encourage others to experiment and save all that pigment dust!
00:00 Introduction
01:05 Tools & Supplies
04:34 Process #1: Grind & Blend Dust Well
06:27 Process #2: Add Dust & Binding Liquids
07:18 Process #3: Cut & Blend into a Consistent Paste
08:54 Process #4: Shape Paste into Sticks
10:19 Process #5: Allow Sticks to DRY before using
10:57 Testing New Pastel Sticks on Sanded Paper
12:08 Comment Below to Receive SUPPLY LIST & INSTRUCTIONS on .pdf
Suggestions for fillers to help your pastels be softer (again, I'm not an expert)
I found one recipe online that uses "precipitated chalk" although I'm not exactly sure what that is.
Here's another suggestion: Chalk is a soft, fine-grained, white calcium carbonate composed of ground-up diatom skeletons. It can be made from slaked and dried lime (sometimes called precipitated limestone) and can be purchased from construction-supply sources. Hydrated magnesium silicate (talc) is often an ingredient in French pastels. Silicon dioxide (silica) is a similar but more refined filler than calcium carbonate and has even less tincture strength. It’s also available in numerous particle sizes. Fillers are not necessary for pastels, although they can provide smoother blends that artists enjoy.
Others include: French talk, Calcium carbonate, or chalk
Just some more possibilities! If you find one that works well for you, I'd love to hear about it!