When I started making drawing sticks from my foraged soil pigments last year, I assumed that I would need to add some additional binder besides the clay that was already in them. I searched for websites on pastel making and found references gum tragacanth - which is the dried sap from several plant species. I purchased a bag of food grade gum, which (according to wiki) is used as a thickener and an emulsifier in food and drugs, and is also used in cake decorating. It came in dried pieces that You re-hydrate with water - quite a bit of water, close to a 20 parts water to 1 part gum (it expands a LOT).
Adding water to the small pieces of gum rehydrated them into globules that would not stir into a smooth gel. I tried grinding the gum pieces into a powder first and then re-hydrating, but just ended up with smaller globules. I went ahead and mixed both of these with my foraged clay soils, adding different amounts - from almost nothing to a lot, and tested the resulting pigment sticks. I found that adding gum tragacanth to my foraged soil pigments did not change the way the drawing sticks turned out - as they were already sticking together nicely because of the clay that is in them naturally. And the addition of extra gum tragacanth did not make the softer pigment sticks any harder, it just made the stick shape and surface of the stick look funny (you can see that in the stick on the right side below, and in the next video). Since gum tragacanth is mentioned numerous places as the traditional binder for pastels, it must be useful to hold pastels together if you are starting from scratch with certain individual ingredients (pigment, chalk filler, etc) but for my purposes, it did not change the way the sticks worked on the paper for the better, so I stopped using it. (If you have successfully used this binder, I would love to hear your opinion/results in the comment section!)
Another binder I found reference to is a compound called methylcellulose. It is derived from plant fiber. It is used as a thickener in foods and cosmetics, as well as for bulk in laxatives. I purchased it in powdered, food grade form, and it re-hydrates into a nice clear gel. The mixing instructions had me heat water to 150 degrees and dissolve the methylcellulose in a 1:8 ratio in the hot water (1 part methylcellulose to 8 parts water), stir while cooling, and then store in the refrigerator. Adding a small amount (1/8 to 1/4 teaspoon, depending on the pigment) of the gel to my pigment stick mixture resulted in a harder stick - one that released less of the pigment to the paper. Being able to increase the hardness of the drawing sticks was useful for a couple of my soil pigments that originally made a very soft drawing stick.
Another ingredient that is commonly added to commercial drawing sticks is natural kaolin clay. I found it to be a good additive for some of my soil pigments that do not contain much clay. Kaolin clay also makes a nice drawing stick on its own, with an almost oil pastel consistency. The only drawback of this particular clay is that it is a very bright white, so it lightens the color of each pigment stick (though that is not always a bad thing, as it has allowed me to make much lighter values of some of my colors).
As I was experimenting with different binders, I happened to forage a neutral tan soil pigment that contained a very waxy clay - it was so fine that much of it went right through the coffee filter when I was capturing and drying the pigment. The clay would not settle, and I had to let it evaporate to dry it out. This neutral toned clay has turned out to be just as useful as the kaolin clay, and I have started adding it to some of my darker soil pigments that need more clay to make a good drawing stick.
The following video clips demonstrates the effects of adding gum tragacanth, methylcellulose and kaolin clay to the foraged clay pigment (I mistakenly say that methylcellulose comes is a plant sap, but it does not, it is processed chemically from plant material). Near the end of the video, I show what the foraged tan clay looks like before it dries.
This second video clip also shows the addition of gum tragacanth, methylcellulose and kaolin clay to a foraged soil. At the end it shows how the foraged tan clay also changes the consistency of the drawing stick. While kaolin clay lightens the pigment sticks, this tan clay keeps the value the same, but tints it slightly more yellow. The first (round) stick in the video was made with just the foraged/processed pigment and water.
The last clip shows the change in the pigment stick with the addition of the foraged tan clay, both with the green pigment (from the last video) and a grey pigment that is also very soft. Both of the sticks are made of the foraged/processed pigmented clay, water, and a small amount of the waxy foraged tan clay.
In this final video, I have started to tint the pigment sticks with other pigments so that I can have a wider range of colors to work with when I use them. The added pigment shown below is natural indigo (the kind you dye with). I use it because it is such a fabulous color - but it is not known to be lightfast for artwork, so anything I create with it will have to be protected against UV light. The base clay is Hudson Green - an upper midwest clay soil given to me by a friend. I add a small amount of kaolin clay because the indigo tends to make the sticks harder, and the hudson green makes a pretty firm stick on its own. You will also see that I have started to make my pigment sticks square, and am stamping them so they can be easily identified (I keep a detailed notebook with the recipes for each stick). I built the little wooden form out of craft wood (pre-cut and easy to find), and use small pieces of wax paper to keep the wet clay from sticking. I leave them overnight between a couple of pieces of drywall - the plaster helps to draw the moisture out, and the boards keep the drawing sticks from warping. I finish drying them on a rack.
I am still experimenting with my foraged pigment drawing sticks, and none of my findings are absolute, There are certainly other additives that work well with different ingredients, and probably some that I haven't discovered that might help the foraged clay soils and pigments I am finding here. I am happy to answer any questions, and would love to hear from anyone who also makes these in the comments below!