I have some gizmos to make and one way would be to silver-solder two
brass bits together, but after that they may be too soft. Is there a
method for *increasing* hardness of brass aside from work-hardening?
For ordinary yellow brass, no. The only way to harden it is by
Some other copper alloys can be hardened by a variety of heat-treating
methods. Beryllium copper can be age-hardened like heat-treatable
aluminum grades. Some obscure copper alloys that contain nickel or
chromium can be hardened by equally obscure heat-treating methods
(transformation hardening; heat-soaked phase transformation).
This is not hobbyist stuff, for the most part, but you can handle
beryllium-copper age-hardening if you have some good temperature
control at around 700 deg. C.