Burnt Sand Mastic.
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.
Traditional Burnt Sand Mastic
Made by roasting sand on a hot plate or in a kiln and was originally mixed with lead based driers and crushed chalk. However for many decades now, alternative natural driers have replaced the lead products. Burnt Sand Mastic is available in a limited number of colours by request.
Masons Mortar Ltd Burnt Sand Mastic as supplied, contains no lead products.
Preparing the joint
New installations should follow traditional practice. Repair work may have to take account of erosion of masonry, movement, poorly fitted previous replacement windows or doors or poorly executed repair work including non hardening modern mastics which require replacement.
Eroded, defective masonry should be repaired to form a neat edge where practicable to maintain the burnt sand mastic joint at a consistent size (approximately 10-20mm maximum). Burnt sand mastic is not a masonry repair material: Where repairs are required to the masonry to maintain a formal edge these should be carried out and allowed to cure properly before proceeding with mastic work.
Joints that have lost their lime backing and where there is clear space between the frame and the masonry must be packed, either with well haired lime mortar or a rot proof compressible filler board or backing strip (Sash and case windows have weight pockets and when filling the gap, care should be taken to ensure nothing enters the weight pocket and affects the free running of the window – For this reason expanding foam should be avoided) The gap must be filled to within 10mm of the plane of the ingo without compromising the weight pocket whilst retaining capacity to support the mastic during troweling on.
The timber frame should have loose flaking paint removed and be fully primed and undercoated before applying the finished mastic, ideally wood work is best fully finished, although a final finishing coat may be left until the mastic has properly set.
Check the absorbency of the masonry, repair or render with water as the mastic is oil bound and high absorbency may occasionally result in bleeding from the mastic into the surrounding masonry.
If the masonry has a high surface absorbency, use tape to protect and mask the masonry to prevent undue surface spread. (Trail samples should be carried out) Although oil stains usually do evaporate over time, avoidance is the best option.
Using a small painter’s fitch, neatly apply a small quantity of the oil drier directly onto the frame and masonry to prime the surface ready to receive the mastic. Keep the primer to the line of the mastic avoid over priming. This can be done up to 1 hour before applying the mastic (weather dependant). Do not allow the oil primer to dry fully, apply the mastic while it is still feels tacky to the touch.
Mixing
As supplied Burnt Sand Mastic is a two part product, the burnt sand, in a tub and the two part oil and drier in a plastic bottle. For each litre of burnt sand add approximately 200 milliliters of oil hardener. This is best added a little at a time, ideally let the sand sit in about 2/3rds of the oil required until it is absorbed fully, and mixing occasionally to ensure all the oil is absorbed evenly. This is best done at least a couple of hours ahead of with the final addition of oil drier added with vigorous mixing. Mastic should drop cleanly of the mixing trowel. – it is not a mortar like consistency, it is pushed into gaps from a mastic box.
(A Mastic box is a tray with at least one side that holds the mastic in place until it is pushed into the gap between the window and the masonry, the side is used to guide the materials into overhead gaps and catches anything that falls back)
Mixed burnt sand mastic can stand in a covered container for approximately 2 hours prior to use, longer in cool conditions – even overnight in some circumstances. Remix immediately before application.
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Application
Using a mastic trowel and box, press the mastic into the space, filling from the bottom of the joint and work up, always lay mastic on mastic, do not work to an open face as this will result in poor compaction, flush the mastic with the face of the Ingo tight to the frame. Where the joint to be filled is less than 10mm it may be necessary to form a neat angled fillet of mastic, not greater than half the width of the visible casing facing. The angle of the fillet from the plane of the ingo should not generally be greater than 150 degrees and in most instances would be flush with the plane of the ingo.
It is important to maintain a consistent margin on the timber frame. Keep tools wiped with an oiled cloth during the application. After filling, starting from a corner, press the mastic trowel tightly against the face of the timber frame and masonry jamb and carefully press and draw the trowel to create a neat regular flush or angled fillet.
Clean away any excess mastic and wipe the finished timber edges on completion. Where tape has been applied to minimize soiling with oil on absorbent surfaces, after finishing the mastic, remove the tape immediately, taking care to ensure that the mastic does not pull away with the tape. There should be no visible lips at the mastic edge on removal of tape.
Mastic should not be over painted; painted mastic deteriorates more quickly and requires more maintenance. If there is a requirement to have the mastic a different colour, coloured mastic is available. Traditional Burnt Sand Mastic as supplied by Masons Mortar Ltd will harden over time, although weather dependant, it should be mixed as and when it is planned for use to avoid wastage.
Terminology.
Oil Drier: Oils containing a drying agent that will cause hardening over time
Mastic Trowel: Long rectangular trowel square sides approximately 18-20mm wide.
Mastic Box: Open ended three sided wooden box with handle to hold mastic while working 150mm x150mm x 50mm deep (a harling trowel can also be used)
Ingo: Inside return from face of wall to window or door frame – forms a 900 angle to the window or door.
Fitch: Thin angled paint brush used for cutting in angles and corners.