Glazing Putty
A linseed oil or acrylic-based compound used to seal the rebate between a single-pane glass sheet and a timber frame in older windows. Putty hardens and cracks with age, allowing water ingress and eventual frame rot. Cracked putty should be raked out, primed, and replaced; on older single-glazed windows it is often more cost-effective to upgrade to sealed double-glazed units.