Looking back through literally hundreds of model engineering magazines, few things have exercised writers more than describing how to judge the hardening temperature for silver steel.
The fashion is to compare this to various food items – notably cherries or boiled carrots. The Japanese swordmakers would heat their work to the colour of the setting sun, just touching the horizon, and that rich orange-red colour – where the light seems almost to come from just under the surface is what I aim for. But the truth is that colours are subjective, and until you know what works for you, you will never be sure. So here is how to be SURE you have the right temperature.
Keep a magnet to hand, and a pair of pliers. As the metal starts to glow, offer it up to the magnet. If the magnet jumps up to the hot metal, quickly remove it with the pliers and heat the metal some more. When the structure of the steel changes to its harder form, the same change will also cause it to lose any attraction for the magnet. Take a good look – whatever colour red it is now is the right one for hardening.
If hardening silver steel you are supposed hold it at this temperature for five minutes for every quarter inch of thickness. If you don't 'soak' the metal for this long, it may not harden right through - in most cases this is not a bad thing, as it means the object will be less brittle, so I rarely keep the work hot for more than five minutes, however thick it is.
A last thought, gauge plate often requires a higher temperature - but the magnet test will still work.