There are several reasons why you should sand the face of your seal stone before cutting your design into it:
- some stones are covered in a wax coating—you may start to cut a seal and realize you are cutting into wax instead of stone
- some stones have rough scratches on the seal face which will show through in the finished seal impression unless they are removed first—these lines are almost always the result of insufficient sanding by the seal stone maker
- some stones may be slightly rounded on the bottom when new
- some stones may have been re-used and the seal face is not perfectly flat or even
The most common method for sanding a seal begins with a flat surface like a countertop; a piece of thick glass also works well. Put a piece of sandpaper down on the flat surface and rub your stone against it. You can sand your stone wet or dry. Dry sanding may seem easier, but clogs your sandpaper more quickly and creates potentially hazardous dust. When dry sanding be sure to wear a good dust mask, especially when sanding soapstone which can be associated with hazardous asbestos dust. Wet sanding is the preferred method and simply requires some way to keep the sandpaper slightly wet. Outside most small seal stone shops in China you’ll see a low stool, a bucket of water on the ground next to the stool, and on top of the bucket a piece of glass with sandpaper on it where the owner of the shop sits and spends their free time smoothing down their stones.
You can move your stone across the sandpaper in a circular or figure-eight pattern, trying to keep it as upright as possible. Avoid sanding back and forth in a straight line as there’s a tendency to unintentionally rock the stone back and forth on each pass over the sandpaper, resulting in a curved seal face. Even with the best of care it can be hard to keep the seal face flat while you sand it. The only way to ensure a flat surface is to use a flat-faced grinding machine—flat laps are grinding wheels for use in lapidary or glass work that have water feeds attached. Unfortunately these can be expensive.
At the China Academy of Art in Hangzhou students in the seal carving classes simply take their stones into the bathroom, place a piece of sandpaper onto the countertop next to the sink and, with water from the sink, periodically wet the paper while they’re sanding. Then, when they’re done, they rinse the sandpaper off under the running water in the sink and wash the stone dust down the drain. Although this method was shown to me by a professor in the art department, it has disastrous consequences. The heavy stone dust slurry collects in the drain and further down wherever the pipe might bend. As it dries out the dust hardens like cement. Even if it doesn’t dry out it builds up until it clogs the pipes and is almost impossible to remove. Don’t wash your stone slurry down the household drain!