I use Formalin, but always with a fan for ventilation. It is an obnoxious odor, and dangerous, but I have gotten yellowing with glyoxal several times so I avoid it.

Chrome alum was used a lot in the past and it works fine, though not as fast as formalin. But there is little or no odor.

Sandy King

Quote Originally Posted by Andrew O'Neill View Post
Which hardener do you use when gelatin sizing the final support paper when carbon printing? Formaldehyde? Will my darkroom end up smelling like the undertaker's?