I do a lot of long exposure photography, in the early days using McSavveny's unusual sequence of developing with HC 110. Today, the HC 110 is a poor developer for that type of extended time in chemistry. There is a host of reasons why Pyro developers are perfectly suited for long times in solution. After trying several notable photogs reciprocity suggestions I have found the Ilford factor system to be quite accurate, possibly a touch too much exposure when you get into more than 60 minute exposure times.
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