Anhydrous or crystalline hypo??

[ColinRH]

I have been making salt prints for some time now and use hypo for fixing. Using Wynn White's method I use a 10% solution. Occassionally, very occassionally, I get very small blotches in the highlights after some 6 - 12 months. I have just realised that the hypo I buy is anhydrous - I am wondering whether I am using anhydrous in a crystalline recipe and therefore over-fixing(which I understand can be detrimental, although I'm not sure in what way).

So the question is; if I read a recipe which contains sod. thio. which do you think the recipe would mean -- anhydrous or crystalline?
Also if the crystals clump in the container does that suggest it is then unusable?

 

[Gerald C Koch]

If you are using the anhydrous form and the formula calls for the pentahydrate then multiply the specified amount by 0.64.

Older formulas usually assume the pentahydrate. Newer recipes should state which form is required.

The asnhydrous lorm will absorb moisture and clump and eventually will set like concrete. However, it is still usuable. Be sure that it is stored in a sealed container and away from moisture.

 

[ColinRH]

Gerald - thank you for that.

I take it that pentahydrate is, perhaps, the correct name for crystalline.
If I AM using anhydrous instead of pentahydrate, am I overfixing and causing myself problems?

 

[Gerald C Koch]

It's hard to overfix particularly with sodium thiosulfate. Ammonium thiosulfate can cause bleaching of the image IF time in the bath is overly extended. There can be a problem with retention of thiosulfate in the paper fibers but this can be helped by use of a HCA.