Sentence:  I dried the extracted leather very slowly on the steam bath ... until the substance was dry enough to <WSD>ash</WSD>. ... I think that the discrepancy in the percentages of "total ash" by method No. 2 and No. 6 is due to this excessive heat required to <WSD>ash</WSD> the leather ... 
Meaning of the word marked with <WSD> tags:  To reduce to a residue of ash. See "ashing".