Chrome gelatin, per Mike's (Down Under) reply.

Clarification:

I am diluting 5 gms of gelatin in 1 liter for a stock solution, then
taking 10 mls of stock and adding to 2 liters waterbath.  IF I use
regular gelatin sub (normally around 100 bloom) I get NO background on my
slides.  However, if I use the 275-300 bloom gelatins (larger molecules)
I can get background staining.  It is the size of the gelatin molecule
(more protein to stain) that causes the background color, not the
concentration that is causing the problem.  I reduce this happening
when presubbing slides, by taking the subbed slides, and dipping them
in formalin to crosslink these large molecules of gelatin.  I find
(for decalcified bone sections only, NOT soft tissues) the 275-300 bloom
works best.

Normally, for daily routine usage, I add a few granules 100 bloom gelatin,
and use this only for standard paraffin section, soft tissue, light
microscopy stains, never for immunostaining (use Plus charge slides only)
and decalcified bone. User beware, you can get background if you add
too much of this granular 100 bloom, and make sure it is dissolved.
On rare occasions and If I have problems with bone, I switch to larger
bloom gelatin, which seems to have better holding power for problem
sections, then live with some background staining.  You can try the larger
bloom gelatins as a few granules added to waterbath,
but it can give some annoying blobs of blue.

Gayle Callis