I was the main bisque sponger during my apprenticeship. It is a much more accurate application than spraying or dipping, and in the case of dusty bisque from sitting around or if you do inlay (which creates dust), it is more efficient in getting the dust off.
Especially in the noborigama bisque, firing temps vary to a large degree. The highest temp was only dull red.
Bisque temp is a factor in absorbancy, but so is bisque thickness. By sponging, you can cause more water do go into soft bisque or thick pots.
We always started glazing the smallest, thinnest pots first. They require glaze with less water in it. As we moved to larger, thicker work, water was added to the glazes. We adjusted up to a dozen times. We always scratch tested the glaze to have an accurate idea about how thick the glaze application was. Its thickness was important over inlay, so that the inlay would not be obscured or undercovered.
Not only that, from 25% to 5% kaolin was added to the ash glaze, depending upon which kiln it went in and what place in the kiln, the hottest spots requiring the most kaolin.
Photo by David McDonald
Warren MacKenzie watched us glaze one time and was amazed by the process, because all these adjustments occur in silence.