Distilled water 750ml
Sodium or potassium metabisulfite 20g
Distilled water to 1 liter
Dilute 1:500 in a 0.666% solution of potassium carbonate (6.66g/ liter), or a 0.5% solution of sodium carbonate (5g/liter).
This is a very concentrated developer with excellent keeping properties, and gives results identical to Hypercat. I give options for using either sodium or potassium compounds, as availability differs, and results are identical.
Handling OA is a little different than handling Hypercat. Hypercat is best dispensed by a measuring syringe, and while this will also work with OA, because of the smaller volumes used, I find a pipet to be most convenient. I bought a precision one like this:
and a latex bulb like this:
The pipet is 1ml and graduated in .1ml, making it easy and accurate to measure out the 0.6ml for making up 300ml of 1:500 working solution for 35mm film, or 1ml for making 500ml for developing 120 film. If you normally make up larger volumes of working solution, higher capacity pipets are available.
There are a number of ways to handle the carbonate. if you always make up a liter at a time, you could add 1 level teaspoon of carbonate to 1 liter of water, and get repeatable results, or you could make up a stock solution (I use a 66.6% solution of potassium carbonate) and dilute it, or make up a ready to use solution of 0.666% potassium, or 0.5% sodium carbonate. The important thing is that the working developer contains 6.66g of potassium carbonate, or 5g of sodium carbonate per liter.
Development times should be the same as Hypercat, provided you're using the same mix.
2.5ml of Hypercat A = 1ml of Obsidian Aqua A
OA 1:500 = Hypercat 1:200, using the same carbonate content for both.
Some starting point development times for OA 1:500 are:
medium to slow films: 9:00 - 12:00, 70F, 10 seconds agitation/ 3:00
400 speed films: 12: -15:00, same as above
Kodak Technical Pan, and other document-type films: 1:1000, 15:00, 70F, agitate 10 seconds/ 3:00
I'll follow with some examples.