bavaria,
Cold browning today, is exactly like it was many years ago.
Browning is simply iron oxide,(rust), once there is a layer produce on the surface, it actually protects the underlying metal from oxygen and additional rust. ie, rust won't rust.
Todays browning chemicals, are essentially just accelerators. While commercially produced, they still do the produce the same results as the ones our ancestors used. Mild acid, urine, etc.
Rust is a function of temperature, time and humidity. We can control those and make the process go more smoothly.
A sweat box, and some wet rags , or even a damp basement work quite well.
I live in Arizona, and achieve excellent results with a sweat box.
All the commercial cold browning solutions work well with the humidity and temperature controlled.
I've used LMF, Homer Danlger, Tru-Brown, all with great results.
Hope this helps.