This is a summary of the state of our solar hot air balloon research. Some of this info is from experiments chronicled in the thread "solar balloon tests in Tbilisi Georgia" much of it concerning the tests of Jeffrey Warren, Lentamental, and myself.
Our designs are based on Robert Rochte's suggestions in the Yahoo Solar Balloons Group:
My revisions to Rochte's design and construction methods are summarized here:
We're trying to powder coat a clear balloon with black pigment, this saves precious weight and makes a stronger balloon. Here is a great chart of pigments to consider:
Iron oxide black pigment is the best pigment thus far. I've gotten mine from industrial supplier Bayferrox, and have excellent results with their Black 330.
Lentamental discovered another black pigment that doesn't really work: bone black, but his balloon flew well despite a lack of pigment. The greenhouse effect works in a transparent balloon! This has great artistic possibilities.
We haven't had good results with charcoal. Pure charcoal is not effective, making the balloons only pale grey.
Formulated, compressed drawing charcoal works better (Char-Kole) but is not much better. Black plastic thinner than .5mil also works, but rips more easily and is only available as cheap trash bags. This increases the number of seams that need to be made.
No one has run tests on industrial lamp black (carbon black). My samples never arrived, and I'm moving on to a helium balloon design I've wanted to work on. I'm happy enough with the performance of iron oxide.
Cabot is an expert in carbon black pigments. They suggested further research on charged pigment coating (tribo charging).