Broken Crayon Enlargement

I was given an extra large crayon enlargement of a civil war soldier with its original ornate frame, catch is, the image fell out and broke off in several places while being moved.

I plan on taking some uncut acid free matting and gluing it back together since the value is essentially gone.

What would be the best glue to use to form a good bond to the mat, but not leak through the image?

 

Probably wheat starch glue because it's water soluble and can be undone. You need to find wheat starch. If you've got a Chinese grocery near you, that's the place to look. Check YouTube for instructions on how to make the glue. It's super easy.