Scott - you can also have a look at Robert Szabo's forum (been around a long time) for answers to a lot of questions:

http://www.cwreenactors.com/phorum/

Also, George Berkhofer's manual is precise, plain-speaking and debunks a lot of the myths asociated with the process:

http://www.collodionart.com/

Mark - the reason is that a collodion positive is, in fact, a thin negative. The dark background (or dark glass) makes it appear as a positive - you can sometimes see the same effect with thin film negatives held against a dark background when the light glances across them.

Regards,
Neil