Restoring a Window with Black Tinted Glazing (Part II)

By Jeffrey Larry

As mentioned in a previous blog entry, the recent replacement of a diamond shaped window pane (Figure 1) from the Cottage’s north elevation raised the issue of whether to use traditional linseed oil putty or a modern putty when setting the glass. The pros and cons of both materials were presented but in order for this issue to be fully considered it is necessary to review the process for preparing and installing both types of window glazing.

The modern putty comes ready to apply directly from the can. The linseed oil putty requires some preparation. (Figures 2, 3, and 4) Remove a small piece from the can and place in the microwave for 10-15 seconds. This will aid in softening the putty. Continue to knead the putty until it is smooth and pliable. If the putty continues to crumble, add a small amount of linseed oil. A paint analysis determined that the original window putty was tinted black. Though it could be done with a powdered colorant, black linseed oil paint was added to tint the putty. A representative from a modern putty company confirmed that the black linseed oil paint could be added to their product to obtain the same results.

After the glass is set on a bed of putty and a glazier’s gun is used to insert pins to hold the glass in place, the putty is applied. (Figures 5, 6)) There are numerous websites dedicated to the process of applying the putty.  The intent of this article is to focus on the putty itself and to compare the preparation and application qualities of modern versus traditional linseed oil putty. A common complaint on many of these sites is that the linseed oil putty is more difficult to apply. On the contrary, the application of the linseed oil putty in this situation was surprisingly easy. Where a modern putty may pull away from the muntin shoulder or be difficult to tool, the linseed oil in the traditional putty seems to have eliminated this problem. In fairness, it does require some trial and error to obtain the right consistency for optimum workability.

To summarize, a modern putty is less expensive, typically available locally, and is ready to use right from the can. Because of chemical additives, it will dry faster than traditional putty but it also dries harder which can cause it to crack over time. The modern putty contains chemical additives which are known carcinogens. The traditional linseed oil putty usually must be ordered on-line, and requires a substantial amount of time to prepare. It is organic, arguably easier to apply and remains flexible.  In keeping with the National Trust for Historic Preservation’s initiatives on sustainability the obvious choice for this site was the traditional linseed oil putty.

Mr. Larry is the Preservation Manager at President Lincoln’s Cottage.
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