I had stumbled upon Jay DeFehr’s website a couple of years ago when searching for an alternative to Pyrocat HD which kept failing for me.  Obsidian Aqua is a simple two chemical (part A) that is highly concentrated and easy to mix. Like most of the cachetol based developers the goal of this developer is acutance and edge effects. While it can be used for rotary you will lose film speed. Jay recommends using 2 or 3 minute agitation schemes.

The formula is written to create a liter of the stock solution, but I did not want to create that much and only want to try a little.  So I set out to mix up 100ml of stock part A. The problem arose that the amount of Cachetol needed does not easily mix in that small amount of water. So I modified it slightly to dilute the stock by half, doubling the water content.  The use of Jays stock mix is 1ml+500ml of part B (6.66g/l Potassium Carbonate or 5g/l Sodium Carbonate).  So I figure I need 2ML part A which is still a tiny amount. I also do not want to mix part B every time and I have  some left over from my Pyrocat HD kits which is 750g/l of Potassium Carbonate.  So my starting developer mix will be 2mlA + 4.4mlB + water to 500ml. Below is the table of Jay’s formula vs my adaptation.

Obsidian Aqua Part A	1Liter	200ml 1/2 strength
Water to start	750ml	150ml
Sodium Metabisulfite or Potassium Metabisufite	20g	2g
Cachetol	250	25g
Water to make	1L	200ml
Use	1mlA + 5mlB* +494mlW	2Aml + 4.4mlB* +500W

*Part B Stock is 666 grams per Liter of Potassium Carbonate or can be made using 500 grams per liter of Sodium Carbonate. Use 12ml B instead of 5ml. You can also just mix Part B working strength as needed using 6.66 g/liter or 5 g/l but still add only 1ml Part A to working strength Part B. If using the Part B from a pyrocat HD kit reduce the amount of part B by 12% or you may need to adjust times due to Activity of the developer.

Starting times for FP4+ at box speed will be 9-12 minutes. ISO 400+ speed films should start at 15minutes or cut box speed. I will let you know what I come up with based on my dilutions and practices in up coming articles.