Note on the time of graying:
(1) The sample in the porcelain boat should be flattened, and the thickness of the sample should not be too large;
(2) When ashing, the furnace door can be opened, and the porcelain boat with the sample on the heat-resistant plate is slowly pushed into the mouth of the box-shaped high-temperature electric furnace, and the sample in the porcelain boat is slowly grayed out and smoked. After a few minutes, when the sample no longer smokes, slowly push the porcelain boat into the hot part of the high temperature furnace, and close the furnace door to burn the sample at 815±15. If there is a coal sample in the ashing process, the coal sample will be discarded and must be re-weighed.
(3) The furnace should have a chimney or venting holes to allow the coal sample to exclude combustion products and keep the air flowing during the burning process.
(4) The control system of the high temperature furnace must be accurate. The temperature rise capability of a high temperature furnace must meet the requirements for determining ash.
(5) The ashing time should ensure that the sample is completely ashed at a temperature of 815 ± 15, but it is also unfavorable to arbitrarily extend the ashing time.