Several people were interested in reading more about the paints we made in Botany. Here is the source I used and here are some details in case you want to make your own. (I thought about making a tutorial, but I didn't because I'm really not sure that I've done this correctly.)
I ground things in my mortar and pestle, but most things were finely ground to begin with. The burnt wood was ground outside with a rock. Everything was air dry when I started.
The gum arabic was purchased at Michael's for about $12.99 (I think). The palette knife was in a two-pack for about $2.99 or so.
After the item was finely ground, I placed it on a plastic tray that grocery stores use for vegetables or deli meats for parties. I found that it took more than a drop of gum arabic to make a paste. Once it looked like it was all combined, I added water. I kept adding water and stirring until it looked like it would yield a fair amount of paint (without giving a paint that was too light).
The paint was then filtered into cups via a coffee filter. You may have to squeeze the filter to speed up the process. But, be careful because it's easy to tear the filter or squeeze the "grounds" up over the top of it.
The paint seems to not spoil too easily. I made it on a Thursday evening and we didn't get to it until Tuesday. It was kept in my cool garage during this time, so that probably helped. The dill paint looked like it may have changed colors a bit, but I didn't compare "before and after" samples. I don't know how light-fast this is or whether it would stay on the paper if the paper got wet.
All in all, this was a fun project that the kids and I enjoyed. Part of the fun was determining what worked well and what didn't (grass, coriander, ginger, parsley). If you try this, I'd love to see your painting!