I love this sort of stuff. I make quite a few of my materials. I buy ground cheap lightfast pigments, ultramarine, mars red/yellow/black and titanium white, and turn them either into paint oil/acrylic or pastels. I do this partly for economy, partly to get a better paint. It means I can turn acrylic medium into the colour paint I require as and when needed. Experimenting with the mixes until you get a pastel that works the way you want is so rewarding. Good luck with your researches.

That is so true that trying to perfect the sticks will drive the person mad. It will be very difficult to make exactly the conte stick, since those sticks are manufactured using high pressure extruders that allow them to use very small amount of water in their recipes, thus they can make harder sticks with smaller amount of binders (CMC or similar).

The screw extruder that I presented in my blog can generate maybe couple of bar pressure, which works fine for wet crayon dough, but doesn’t work with dryish mass that would produce the perfect conte sticks. The professional stick makers use hydraulic extruders that produce about hundred times more pressure. I’ve been thinking about making new extruder by taking parts from machine vise and designing it so that it could give enough high pressure, but haven’t had the motivation or enough time to make it.

If there is no extruder, then another option could be to make a steel (or hardwood) mold and use it to compact the crayon dough to stick shape. Ideally the dough should contain so little amount of water that it just barely holds together when squeezed between fingertips.

Yes, compression is the key to good quality sticks. My screw extruder was nice in a way that it massaged the air out of the mass, but it required just the right moisture level to work correctly, so it was sometimes difficult to operate. And the moisture level depended on the pigment. Graphite was slippery and required more water (in the stick recipe), titanium oxide was the opposite and worked like a dream with tiny amount of water. Iron oxide pigments were bit like titanium oxide in a way that those could be worked with quite small amount of water.

It might be difficult to mix the mass with enough low water content, so one may have to start with higher moisture level, mix everything together, let it dry some time, mix it again and then compress to the stick shape. Ideally the loose mass should feel like moist powder.

I have no experience of gum tragacanth. Gum arabic didn’t work, since it moves with the water and accumulates to stick surface during the drying, which makes the surface rock hard. CMC does not behave that way, so I have preferred it. The real conte sticks also contain CMC (+kaolin + pigment). Such binders should be used very sparingly, because it happens easily that the binder binds the materials too well and the sticks don’t leave any mark on the paper.

Bentonite should improve the mass working properties, since it makes everything more slippery and plastic, so one could use less water. It should be used sparingly just like the binders. Kaolin should be the main clay that forms the stick body, bentonite makes the mass easier to work, and the gum/CMC binders will add strength to the sticks.

One could also use tiny amount of deflocculant chemicals like soda ash to further reduce water content, but I’ve avoided those since those would affect the stick pH and could produce archival issues. It is unlikely that those would cause problems (the clay pH is high-ish and soda ash may not change anything), but I’ve been too lazy to try those.