Masons Mortar Ltd
77 Salamander Street
Edinburgh EH6 7JZ

Tel: 0131 555 0503
Fax: 0131 553 7158

Masons Mortar Ltd
Unit I, Woodville Court
Woodville Street
Glasgow G51 2RL

Tel: 0141 445 4812
Fax: 0141 445 8898

Masons Mortar Ltd
Block 2
Woodend Industrial Estate
Cowdenbeath, Fife

Tel: 01383 514 460

Masons Mortar Ltd
Unit 11
Brough Park Trading Estate
Fossway, Newcastle upon Tyne

Tel: 0191 908 9234

Burnt Sand
Burnt Sand Mastic.

Traditional sash and case windows and door frames

Traditional windows and door frames were not and should not be fixed in any way to the outside masonry rather they are wedged and held from the inside by battens for lath or plaster or the window shutter casing or on the hard with plaster tight up to the frame. They were 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. Available in a limited number of colours by request.
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 san mastic joint at a consistent size (approximately 20mm). Burnt sand mastic is not a masonry repair material: 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 such as Flexcell, in all events the gap must be filled to within 10mm of the plane of the ingo.
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 timber frame should have loose flaking paint removed and be fully primed and undercoated before applying the finished mastic. 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 painters 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, apply the mastic while it is still feels oily or tacky to the touch.


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 ltr of burnt sand add approximately 200ml total of oil hardener is added a little at a time, mixing carefully at each addition until the mastic is worked into a thick putty like consistency and comes cleanly of the mixing trowel. It should be possible to turn virtually all the material into a large ball with a single flip or the trowel. Allow to stand in a covered container for approximately 2 hours prior to use to ensure that all the oil and hardener has fully integrated with the burnt sand. Remix immediately before application.

Using a mastic trowel and box, press the mastic into shape in the box and lift with the trowel, 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 exceeding a 45 degree angle. 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, paint deteriorates relatively quickly and requires constant maintenance.
Once mixed with the oil drier and re-mixed prior to use, mastic will begin to harden, therefore mix only that which can be used within 4 hours. In normal weather conditions a noticeable hardening will take place within 24 hours.


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: Inband return from face of wall to window or door frame

Fitch: Thin angled paint brush used for cutting in angles and corners.