Restoring traditional timber sash and case windows in Scotland is a specialized process that preserves the historical integrity and weatherproofing of heritage buildings. This maintenance focuses on two critical materials: burnt sand mastic for the masonry joints and linseed oil putty for the glazing. Unlike modern synthetic sealants, these traditional mixtures are breathable and flexible, making them perfectly suited for the damp, fluctuating Scottish climate and the natural movement of historic stone and timber structures.

The Removal and Preparation Phase

The restoration begins with the meticulous removal of aged, brittle putty and failed sand mastic. Using traditional hacking knives and chisels, craftsmen clear the glazing rebates and the external gaps between the window frame and the masonry. It is vital to avoid splintering the historic timber or damaging the original glass. Once cleared, the surfaces are cleaned and primed with linseed oil or a dedicated sealer. Deep gaps are often packed with haired lime mortar to provide a solid base for the new mastic.

Applying Traditional Burnt Sand Mastic

Burnt sand mastic is a unique Scottish mixture of roasted sand and boiled linseed oil, typically mixed to a “damp brown sugar” consistency. It is hand-applied using a specialized pointing trowel to form a neat, 45-degree beveled fillet between the timber and the stone. This material is prized for its tenacity and slow-hardening properties, which allow it to accommodate structural movement. Because it is breathable, it prevents moisture from becoming trapped behind the frame, thereby protecting the masonry from decay.

Re-glazing with Linseed Oil Putty

For the glass-to-timber seal, traditional linseed oil putty is worked into the glazing rebates. The putty is kneaded until pliable and pressed firmly around the panes to ensure a watertight bond. A smooth, angled finish is achieved using a putty knife, aligning perfectly with the internal timber profile. This process requires patience, as the putty needs several weeks to form a protective skin before it can be painted. This traditional seal remains flexible for decades, accommodating the natural expansion of the wood.

Final Finishing and Long-Term Maintenance

Final protection is provided by high-quality, oil-based paint applied once the materials have sufficiently cured. The paint should overlap the glass by a few millimeters to create a secondary weather seal, often called “painting the line.” In Scotland, using these historically accurate materials is essential; they are chemically compatible with heritage structures and are easily reversible. Regular inspection and renewal of these seals every few decades will ensure that these iconic windows continue to function and protect the building for generations.