Are there are good materials like instant coffee, for making homemade natural inks?

Question

I recently figured out that if I apply a little bit of white vinegar to instant coffee, and add some honey, I can make an ink that doesn't remain as clumps like carbon based ink like home made india ink.

Are there other materials that dissolve in boiling water, or will I ultimately have to focus on straining these?

My current purpose is to use these as shading ink, for when I'm not doing line art with a Bimoji brush pen. I also like using a Sumi brush for certain purposes.

I tagged this as skill swap, but it could be either that or tools and suppliers. Though my focus is on creating home made inks for calligraphy and sequential art.