Interesting that while dials were not marked to indicate that radium had been used, the boxes for the dashboard timers were marked "Rad", at least in certain periods.

Here is such a box, along with the relatively early timers, which used radium. The "Pat. Appl. For" [patent applied for] on the dial confirms that this is an early pair.

Jeff

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: Radium was used in some applications until then and actually even
: later, for military watches and the like. However, the health
: issues had long been a concern by then and it was very rarely
: used in civilian watches at that late a date. The introduction
: of T markings was thanks to similar concerns - tritium had long
: been in use by then but hadn't had safety markings mandated.
: You'll see radium has to be marked on the dash timers already a
: few years before this. Civilian watches would have had to be
: marked too, but it had dropped out of use by almost all in the
: 60s hence you don't see it often.

: The brown colour isn't an indicator for radium, that's a bit of a
: misunderstanding of how lume works. The "active"
: materials (in this discussion, radium and tritium) are used as a
: source of radioactive decay, they don't directly produce the
: glow. The particles the decay produces are used to excite a
: luminescent material, typically compounds of phosphor, and it is
: the choice and mix of phosphor compounds that give the colour to
: the lume, both out of the box and with age. Consideration to the
: mix as regards reaction with water is necessary too, some absorb
: too much and later dry out and crumble.

: Tritium would still be at 12.5% effectiveness after 36/7 years (or
: 3 half-lives) so would still be active enough for residual
: luminescence. The delimiting factor is as often how stable the
: phosphors have been in that period - it's no use still having an
: active tritium source if there are no phosphors left to excite.