M
Hi. I suspect that the problem is with the gelatin that you are using. I don't prepare chrome-gelatin anymore but we used to use silver or gold leaf gelatin from Belgium as  this was the highest purity gelatin available.
Gelatin batches can vary considerably even if they have the same bloom number.

Prepare 2 solutions
Solution A
2.5 gms silver leaf gelatin
350 ml. distilled water
Dissolve with gentle heat.

Solution B
Dissolve 0.5 gms chrome alum in
150 ml distilled water.

On day of use mix solution A and B and filter.
We used to dip slides at least twice in this solution, allowing to dry between each coating.
Another source is the glass slides that you are using. Are you cleaning these yourself?
We used to clean slides in nitric acid, wash in water and then in glass distilled water and dry in a dust free incubator before dipping.
Barry

Mary Stevens wrote:

> Hi all,
>
> For years, I've been using the same Gel-Chrome Alum Concentrations/methods to coat slides for undecalcified bone (for MMA) - lately we are getting an overall background staining on the slides (not clumping, but what appears as too much gelatin.)  The obvious thought is to decrease the 1% to 0.75% gelatin - but I'd like to hear what solutions you are using and your thoughts on why we are seeing the background increase recently (solutions have been made up several times - so I do not believe there is a technical error in concentrations.)
>
> Our recipe is:
> 1000 mL of 1% gelatin, Bloom 275
> 38 mL of 4% Chrome Alum
>
> Thanks, and have a great weekend.
>
> Mary