The Due-Diligence Interview: Ten Questions for Period-Glass Conservators

Historic glass invites confidence. It has survived a century or more, and the temptation is to assume that anyone with a van, a ladder and a roll of lead knows how to care for it. That assumption is costly. The difference between sympathetic repair and irreversible damage is often a matter of method, and method is revealed by questions. A competent conservator will welcome them; a careless one will dismiss them.

Why the vocabulary matters

Homeowners are not expected to know the difference between lead came and copper foil, or between traditional putty and a modern sealant. But you can ask for explanations in plain English. If a contractor cannot explain what they propose without hiding behind jargon, or cannot say why one method is preferable to another, pause. The craft has a vocabulary, but it should not be used as a barrier.

1. Who will actually carry out the work?

Ask whether the person quoting will do the work or subcontract it. Ask to meet the conservator who will handle the glass. If panels are to be sent away, ask where and to whom. A vague answer is a red flag. So is a salesperson who cannot name the craftsperson.

2. What is your diagnosis, and will you put it in writing?

Ask for a condition report with photographs before work begins. It should identify failed lead, corrosion, broken glass, paint loss, previous repairs and any structural concerns. A written diagnosis protects both parties and gives you a baseline for future maintenance.

3. Will you retain the historic glass?

Original glass has value. Ask what will be kept, what will be replaced, and why. Replacement should be a last resort, not a convenience. Be wary of anyone who promises a brand new window as though that were an improvement. Historic glass is not improved by losing its history.

4. What lead profile and width do you propose, and why?

The lead came must suit the panel. Ask about profile, width and alloy. A heavier lead is not automatically better; it may distort or overload the window. The answer should refer to the existing work and the exposure of the window, not to a generic specification.

5. How will you handle putty and cementing?

Putty is not a disposable detail. Ask whether traditional linseed-oil putty will be used and how it will be worked. Modern sealants can trap moisture or fail to support the glass correctly. The method should match the panel, its age and its position in the building.

6. How will you clean and protect painted glass?

Paint on stained glass is fragile. Ask about cleaning agents, brushes and whether the panel will be flushed. Abrasive cleaning or harsh chemicals can remove historic paint. Request references for painted work, not just plain leaded lights.

7. How will you repair breaks and losses?

Ask whether breaks will be mended with copper foil, lead or new glass; whether new glass will be matched; and whether repairs will be visible. Sympathetic repair does not mean invisible at any cost. It means honest, stable and reversible where possible.

8. What records will you make?

Ask for photographs before, during and after the work, a diagram of panels, and a record of materials used. Records support insurance claims, future repairs and the sale of the house. They are part of the conservation, not an optional extra.

9. What are your insurances, accreditations and references?

Ask for public liability and works insurance, relevant accreditation, and references from comparable period projects. Check them. A conservator who cannot provide evidence of insurance should not be handling fragile historic glass in your home.

10. What happens if you uncover concealed decay?

Ask how variations are priced and approved. A fixed quote that ignores hidden rot or corrosion is not a bargain. Agree a process in writing: who decides, what it will cost, and how the work will be documented.

Red flags that should end the conversation

Pressure to replace rather than repair. No written method or condition report. Inability to name subcontractors. Refusal to provide references. Promises of invisible repairs. Cash-only requests. Dismissal of your questions as unnecessary. Any of these should make you pause, and several should make you walk away.

A final test

The best conservator may not be the cheapest or the most talkative. They will be the one who can explain why a particular lead, putty or repair is right for your glass — and who is willing to put that explanation in writing. In period glazing, method is the message. Choose someone whose method you can trust before the first panel is lifted.