Sentence:  THE FRONT DOORS OF MY STOVE HAD WINDOWS of <WSD>isinglass</WSD> (mica) which needed replacement. While impervious to heat, <WSD>isinglass</WSD> is quite fragile and one must avoid touching these panels when using the doors. The panels are held in place by a frame bracket screwed to the inside of each door. Fortunately, a supply of extra mica panels was found in the house. 
Meaning of the word marked with <WSD> tags:  A thin, transparent sheet of mica (probably from its similarity to true isinglass).