Rollei is partially right about absorbent materials and the need for gelatin...for some of them. If you're talking about fabric or something that's similarly porous, then yes, definitely use it. Papers can be another matter, again depending on how porous it is. Obviously with a non-porous paper, more of the liquid emulsion is going to remain on the top, and thus deliver maximum response. Most papers today are non-porous enough that gelatin isn't 100% necessary, as only a small amount of the emulsion will sink into the paper fibers. Fine-art papers can be a different story...there are some of them out there that are so absorbent that they almost act like sponges. You really need to be careful if you're using one of those.