Has anyone here any experience with overcoating cyanotype? A recent thread in another part of APUG about photo postcards got me thinking about cyanotype postcards, which I've heard of -- but I'd be concerned about postal abuses damaging the image, not to mention having to spend a lot of hours explaining cyanotype to the USPS inspectors if they happen to connect the beautiful image with the scary-named chemicals that are used to make it (to the ignorant, any chemical with "cyanide" in the name must be terrifically toxic, right?). Overcoating the image would help in both senses, by protecting it from liquid splashes, alkali in the environment, rubbing off the paper, and so forth, as well as "protecting" postal personnel from contact the the "hazardous" Prussian Blue pigment...

Question is, what overcoat works? Artist's fixative spray, in my experience with drawings when in high school and college, isn't all that great (and neither is Aquanet hair spray, though it's both better and cheaper than fixative), and I have no idea what they'd do to a cyanotype (though that part's easy enough to find out). What about a brushed or air sprayed coating, either a lacquer of some sort (model airplane butyrate dope, say) or a urethane product (like thinned Varathane varnish), or even shellac? Gelatin, albumen, or starch? Something else I'm not thinking of?

You may wax it. it will protect the print.

I think I've read that these prints need to breath or need exposure to air.

As far as coating I would probably experiment with an acrylic or petroleum based varnish. Hmmm...I wonder what damar varnish would look like over a cyanotype or vdb?

Cyanotype postcards were often produced and mailed 100 or so years ago, so they may survive the postal system. You could make two or three and mail them to yourself to see what, if any, damage they are suceptable in the 21st century. As to what the USPS inspectors will wonder, just describe your postcards honestly as blueprints ;)

See this thread about using wax.

http://www.apug.org/forums/showthread.php?t=20266&highlight=wax+medium

Menacing,you're correct in that exposure to oxygen is required for a new print to reach the full richness of the blue (oxidation of the Prussian blue deepens the color -- though a peroxide treatment during development will do the same job before the print is even dry). Beyond that, air isn't really an issue, and unless both sides of the print are coated, I would expect more than enough oxygen to diffuse through the paper to keep the color up to snuff.

Wax, damar varnish -- and acrylic painting medium occurred to me after I posted. I'll obviously have to make some cyanotypes and test some stuff. Exposure shouldn't run over an hour, now that winter is coming on... :P

I think Menacing may have a point - light will reduce the Prussian blue to Prussian white, which is why cyanotypes fade in light. They normally recover by oxidation when placed in shadow, but the wax or other coating might interfere with that. I can try with my coated cyanotype if I get a chance.

If you coat both sides, there might be a big problem with oxygen reaching the Prussian (white/blue) layer, but I don't know of any watercolor paper that's gas impermeable -- so oxygen will still reach the pigment through the back side, as long as that side isn't coated. Recovery after light overexposure might be slowed, but shouldn't be precluded.

If your overcoat doesn't shrink, causing curling of the paper, there's no reason to want/need to overcoat the back...