The original mastic for junctions between masonry, render and wooden windows and doors, also used extensively for leaded or metal framed windows up and down the length and breadth of the United Kingdom and Ireland for at least 200 years, Burnt Sand Mastic fell out of favour because it required site mixing.
The emerging DIY market developed more user friendly, but actually significantly less durable of the shelf products with a much shorter life span, an often repeated short term solution, now clearly recognised as ineffectual and not fit for purpose without constant maintenance.
Traditional windows that are set behind a masonry rebate are not fixed in any way to the outside masonry; rather they are wedged and held from the inside by battens for lath and plaster or plaster on the hard with plaster straight into the woodwork or by window shutters, in all cases the actual window frames are free.
They were often face bedded around the frames with hairy lime plaster to ensure full contact with the rebated masonry detail, and subsequently pointed up externally with Burnt Sand Mastic to form a long lasting, completely wind and water tight flexible joint between the outside masonry and the timber frame. This method of fitting timber frames to openings in traditional masonry buildings has never been surpassed and should not be altered.