To perform gravimetric analysis, a constant weight porcelain crucible should be prepared first. The procedures are as follows,
1. A porcelain crucible and lid are cleaned with deionized water and then placed in air oven at 110 ゚C for 15 mins to dry.
2. The porcelain crucible and lid are placed into a desiccator and cool it to room temperature.
3. Weigh the porcelain crucible and lid using an analytical balance before heating.
4. Heat the porcelain crucible and lid to dull redness for 15 mins with a Bunsen burner or in a muffle furnace at 800 ゚C.
5. Cool the porcelain crucible and lid in a desiccator to room temperature.
6. Weigh the porcelain crucible and lid.
7. Repeat the steps 4 to 6 until a constant weight is obtained.
What I confused is the differences between steps 1 and 4. I know step 1 is to evaporate the deionized water on the porcelain crucible and lid. But how about step 4? What is its purpose?