Quote Originally Posted by john_s View Post
Thanks Mark. About metaborate:

tetrahydrate NaBO2-4H2O
octohydrate Na2B2O4-8H2O
You might have misread something here. There are four compounds appearing in literature:
  1. NaBO2 * 2 H2O
  2. NaBO2 * 4 H2O
  3. Na2B2O4 * 4 H2O
  4. Na2B2O4 * 8 H2O

Note that compounds 1 and 3 are effectively the same thing, and the same applies to compounds 2 and 4. Both compounds 2 and 3 could justifiably be called tetrahydrate but are certainly not the same, and that's where the confusion comes from. Mark has written about this and linked to relevant information. Make sure you know which compound you have and use the amount recommended by Mark for that amount. You dev is likely going to develop film even if you pick the wrong amount, but if you want to get the same good results that Mark achieved with his formula, you better get the metaborate amount right.