Early putty was a mixture of lime and/or white lead mixed with linseed oil. Because asbestos fibers often were added early in the 20th century, it’s important to wear dust protection when you remove hardened old putty. Safe linseed-oil putty—glazing compound—is available today. A little practice will make you an expert glazer.
The Right Way
DO USE GLAZING COMPOUND, AKA PUTTY
Traditional glazing is a simple process: all you need is a putty knife and a handful of well-kneaded glazing compound. Using the putty knife, you’ll easily create perfect putty bevels against virtually any muntin profile. You’ll be able to cut sharp corners and leave an overall neat appearance that’s ready for priming in a week or sooner. Cleanup is quite simple. If the window is properly maintained, you can expect the glazing compound to last for many, many years.
The Wrong Way
DON’T USE CAULK AS PUTTY
Caulk may seem like the easier approach to sealing window glass into sash, but it has drawbacks, both cosmetic and functional. The sash rabbet (the little shelf that the glass sits on) varies in size, and a bead of caulk won’t accommodate that. Caulk is difficult to control and smoothing the bead is challenging, especially in as fine a job as glazing. Caulk shrinks, leaving a concave bed that may hold water. Caulk doesn’t have the long lifespan of glazing compound. Furthermore, high-performance caulks with strong adhesion (like silicone caulk) are so tenacious, the effort of removing them later may break the glass. Caulk has to be removed mechanically and it’s not uncommon for it to take wood with it, leaving a ragged edge.