Re: Hot blueing
As mentioned by the others, hot bluing requires a little more than cold bluing if you want to do the job right. With that said, I've seen more than a few guns come through the shop that had strip-&-dip jobs.
Not much equipment is really needed other than bluing tanks, a buffer, barrel spinner, belt sander, emory cloth, and a file. Of course the file is only needed if you want to do the job right.
Bluing salts are really nothing more than fertilizer. I can remember two of them off hand, sodium nitrate, sodium hydroxide, and possibly sodium nitrite. No restrictions on disposal, but they are miserable to work around and especially if you get them on your hand or a leather work boot.
Most hot-blue jobs I've seen come in neglect draw-filing and just run the barrel and action on a buffer or belt sander to the desired finish (the strip). They dip them and presto, all the pits show up (the dip). The strip and dip jobs usually end up rounding off the parts that should be square and dish-out screw holes.
My dad still blues at his shop but its really hard to find anyone willing to pay for the job due to strip and dippers. When he blues, he is probably only making a few dollars per hour. If he charged the actual price by the hour it wouldn't come out less than $300 to do a gun. Strip and dippers seemed to always charge around $80. Of course you get what you pay for. Unfortunately, few people really know what a good bluing job looks like.
Also, contrary to popular belief, a good gunsmith can't effectively blue 8 or more guns at a time unless they have multiple sets of tanks and multiple hands. I think the most we have tried at one time was 4.
Different methods are needed for different type of guns as well but few that do hot bluing realize this. I've seen 2 high dollar double barrels come into the shop ruined due to a "gunsmith" not knowing that you can't hot blue a double barrel with soldered barrels.