Homebrew PCBs

>But any way you cut it, ammonium dichromate is a nasty substance that just 
>loves to crosslink collagen; of which there is plenty in the human body. 
>So be careful with this stuff.


     Yes, quite. If you've seen the "Erin Brockovitch" (sp?) movie, the 
substance that was getting into the groundwater (and ruining people) was 
hexavalent chromium, which is the ammonium, potassium, sodium, etc., 
dichromates.

     I believe that the information on chromated gelatin over at the 
holography site includes something regarding mixing waste dichromate 
solutions and wash water containing dichromates with used photographic 
developer, which changes it to an insoluble form. This drastically 
reduces it's impact.

     I used to be involved in the corporate use of this process on a 
relatively small scale, and they eventually had to stop and find a 
replacement. An industrial silkscreen-making goop (diazo?) ended up being 
the thing, it dropped right into the process (being sprayed on, resisting 
acetone, coming off with bleach, etc). The toughest part was figuring out 
how to measure it out for amounts much smaller than the one-gallon kit.

     In the above process, I was aware that a competing company would add 
Elmer's white glue to the mix when they had adhesion problems with the 
plain gelatin/dichromate mixture. I'm not sure if they realized that the 
glue was also crosslinking.

     The green pigment may not be the best choice for sensitizing (the 
base mixture requires UV). Look on the holography site for information on 
red-sensitive DCG - although then you'll have to use a dark green 
safelight, which gives rise to the phrase "working in the green 
darkness". It's probably easier to come up with enough UV, I expose UV 
sensitive resist with a cheap 500 watt halogen with the UV shield glass 
removed (~4 minutes at 18 inches through an old glass-topped photo 
contact printer).