Woodstove Film on Interior Glass

What the film actually is

Wood combustion puts fine particulate, tar compounds and moisture into the room air, even from a well-sealed stove, and more from an open fireplace or every time a door is opened to reload. Those compounds condense on the coolest surfaces available. Window glass is the coolest interior surface in most rooms, so it collects the heaviest deposit.

Why it smears rather than wipes

Because it is tar-based rather than particulate, dry wiping drags it into streaks and a damp cloth spreads it. It needs a cleaner that breaks the oil down and then a rinse that removes it rather than redistributing it. This is the most common reason people conclude their interior windows cannot be got properly clean.

It is mildly acidic on other surfaces

The same residue lands on frames, sills and window coverings. On painted wood it dulls the finish over time, and on fabric coverings it holds odour. Cleaning the glass and ignoring the frame usually leaves a visible line where one was cleaned and the other was not.

Timing it for the end of the season

There is little point clearing it in January with three more months of burning ahead. A single thorough interior clean at the end of the heating season removes a winter of accumulation at once, and it is the highest-value interior booking of the year for a wood-heated house.