Restoring stained glass with Holy Well Glass

Stained glass was in widespread use in Britain from the medieval period until Victorian times. This glass, from St Winnow's church in Cornwall, dates from the 15th century. St Michael's face clearly shows the damage caused by centuries of condensation eating away at the glass Photograph: Sam Frost/Sam Frost

The view through a microscope of the reverse of a piece of medieval glass Photograph: Sam Frost/Sam Frost

Steve Clare, the company's founder, shows off an invisible repair to a broken piece of glass. Modern epoxy adhesives mean that old repairs using lead can be replaced and the integrity of the window's design restored Photograph: Sam Frost/Sam Frost

Clare shows off one of the pieces of a medieval stained glass window his team is working on Photograph: Sam Frost/Sam Frost

All work done to every window is meticulously recorded so future restorers will know exactly what has been done and when Photograph: Sam Frost/Sam Frost

An awful lot of latex gloves and brushes: some of the tools of the trade Photograph: Sam Frost/Sam Frost

Clare watches as conservators Helen Bree and Gary Graves carry in a window, like a patient on a stretcher Photograph: Sam Frost/Sam Frost

Bree and Graves at work on a window. Each pane is carefully dismantled and the glass painstakingly restored using special fluids, brushes and scalpels Photograph: Sam Frost/Sam Frost

New glass is used only as a last resort, where old pieces are beyond repair. Colours and paint are used to blend in the repair, but new pieces are date-marked to indicate their presence – there is no attempt to deceive Photograph: Sam Frost/Sam Frost

A sheet of handmade red glass. The colour comes from the addition of gold oxides to the molten glass and will not fade Photograph: Sam Frost/Sam Frost

Any new detail painted on to the glass during restoration will need to be fired in a kiln Photograph: Sam Frost/Sam Frost

Once the glass has been cleaned and any broken pieces repaired, the window is reassembled. If the original lead is present it will be reused where possible, but many windows have been releaded several times and new lead is commonly used Photograph: Sam Frost/Sam Frost

Greville Thorne works on the leading of a window Photograph: Sam Frost/Sam Frost

A window is pinned in place while being reassembled Photograph: Sam Frost/Sam Frost

Different sized leads Photograph: Sam Frost/Sam Frost

An oyster knife, used for easing the glass into the lead Photograph: Sam Frost/Sam Frost

Charts and records of some of the pieces Holy Well Glass have worked on Photograph: Sam Frost/Sam Frost

Off-cuts and glass mementoes and samples and test firings of different glass paints line the windows Photograph: Sam Frost/Sam Frost

Clare stands beside a window awaiting restoration Photograph: Sam Frost/Sam Frost