There is probably no means by which leaded glass, because of the innate character of lead as its skeleton, can resist its propensity to bend, bulge and sag. Evidence of these occurrences does not necessarily foretell disaster or immediate collapse. Bulging does not necessarily indicate the need for action or re-leading.
There are three basic stages through which stained glass passes on the way to requiring repair;
1. Bulging, bending and sagging
2. Loss of putty and breaking of solder joints
3. Unhousing of the glass from the lead
The points at which solder joints break depends on the materials used.
Since lead, compared with solder, is a resilient material abutting the more resistant solder, breaks will occur most frequently at the junction of the solder with the lead.
With zinc, the situation is reversed. The zinc is of greater resistance than the solder. As a result the break most often occurs on the solder at the point of the zinc junctions.
It is the very existence of resilience in lead which responds to the expansion and contraction of glass that permits the more healthy survival of the glass over the less sympathetic accommodations of either zinc or copper foil. Leaded glass, unlike any other medium, has the unique capability of having its skeleton (lead) replaced, when the need arises, without damaging its body (glass).