A window would be great. to date we haven't found a glass that will handle the heat and cold. all of them just shatter... well except the ones that blow up.
Need more info?
Ernie and Erica
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I believe that Ernie is correct in that any installation in the firebox will result in too much heat or thermal shock. The way I've seen it done successfully is to create a "viewing tunnel" from the side or back of the feed tube, about 1' long, to the window. That way you are looking into the bottom of the feed tube, but the glass is a long way from the heat and out of the path of air. I think that as the clay dries it will be hard to get a great seal, but a little airflow into the burn from there shouldn't be a problem.