Homebrew PCBs

Well, after babbling away about how good my el-cheapo ML-1210 laser 
printer makes transparancies, I made a little board (4"x4") with 
0.008" traces on 0.020" centers over half of it and the other half 
as a ground plane. The transparancy came out great, no pinholes etc. 
Exposured and developed, only found one small break (and a 
corresponding piece of dust on the transparancy). Quick jab with my 
sharpy and its ready to go.

The shocker was when I went to turn on the heater for my ferric 
chloride. I hadn't realized it but its been months since I etched a 
board - instead of being a translucent dark brown, it was an opaque 
greenish brown goop! I didn't have any more on hand and it was a 
sunday so..., topped it up with water, heated it up, and etched 
anyway - worked perfect! In fact, this was about the best etching 
I've managed (probably because I exposed longer than needed and got 
more aggressive with brushing in the developer.

Why would it turn green? Something growing in there?