A standard method that we used to calibrate the evaporation rate of the 100C oven. The procedures are as follow:
1. weight a empty dish, as M1.
2. put ~ 300mL of water to the dish and weight, as M2.
3. put it in the oven and note down the time, t1
4. after a period of time, t2, take out the dish to cool and weight again, M3.
The rate = loss of water/ time escape.
NaHCO3, Na2CO3, CaCl2,...are quite common and obtainable from store.