I believe the critical temperature is 727 degrees Celsius, if I'm not mistaken. If one has an optical pyrometer (which I don't), one could measure the temperature from a safe distance. Also, a bit off the subject, the Verhoeven mentioned is the same one that validated the re-discovery of true Damascus steel techniques by Alfred Pendray. I read his papers on the studies and they were all very informative - recommend them to anyone who has the time/capabilities/equipment to attempt making true Damascus items vs. normal pattern-welded items.

Critical Temp also depends on the alloy we are using at the time, they are not all at the same temp for proper hardening.