Case study | Laser quest

A laser formerly used to treat skin blemishes is being applied to remove darkened and insoluble restorations from delicate historic decorative surfaces at the National Trust’s new Royal Oak Foundation Conservation Studio at Knole country house, in Kent.

The removal of old repairs applied to painted or gilded surfaces has often been difficult to achieve when they have similar solubility parameters – meaning solvents that can helpfully remove an overlayer can potentially also negatively affect the underlying original.

Following promising trials on gilded surfaces at the City and Guilds of London Art School, a Fotona (Dualis XS) Er:YAG laser was donated to the studio by laser expert Ed Teppo. A controlled area was established for using the laser, and staff and contractors were trained in its safe use.

The National Trust’s approach to looking after the objects in its care seeks to fully consider their often-complex context of collection and setting. This means it is important to retain signs of age, use and change. Therefore, treatments in the studio require discussion with property staff, curators and other specialists before any change to the object’s appearance is attempted.

Removing discoloured restoration

The conservators have been developing techniques for removing the stubborn top layers of paint with the laser to reveal the earlier surfaces beneath. The laser is used to safely remove non-original layers that had not been possible using solvents alone.

The studio’s work has concentrated on the particularly difficult problem of safely taking off old repairs made using metal flake paint – commonly referred to as bronze paint – from gilding. Conservators have been able to compare the results of cleaning using only solvents with those using the laser.

Despite the laser’s low energy in comparison to others used in conservation, especially on stone surfaces, the cleaning action was initially difficult to control. Therefore, an energy attenuator was made to reduce the output of the handpiece by 75%.

This dramatically enhanced the effectiveness of the laser in treating delicate and thin surfaces such as gilding. Various approaches were taken for pre-wetting the surface to keep the laser energy’s heat close to the surface, by using different laser parameters and solvents for pre-wetting and clearing, and mechanically thinning layers disrupted by the laser.

As part of the 125 Treasures of the National Trust project, which celebrated the charity’s 125th anniversary, an important set of nine English chairs from the 1630s were treated in the studio. The chairs had remained at Petworth House over the centuries and there had been attempts over time to redecorate and touch them up. The usual technique to remove overpaint on objects is by using carefully chosen solvents applied to small areas at a time, but one of the Petworth chairs demanded a different approach, which the laser fulfilled.

Collaboration and dissemination

The studio’s work inspired a special issue of the Journal of the Institute of Conservation dedicated to the use of Er:YAG lasers in conservation in February 2020. The studio now plans to continue collaboration and research with museums and universities to further develop the laser’s application for treating a variety of materials. It also hopes to look at the use of the laser for the conservation of historic paintings, especially in work where overpaint is similar to the original paint and difficult to remove safely by traditional techniques.

Using the laser has facilitated the partial or full removal of disfiguring later layers more safely than by using solvents alone. Another advantage is that using Er:YAG lasers to treat an increasing range of materials means that fewer solvents are needed.

Gerry Alabone is senior conservator (furniture and frames) and Emma Schmuecker is studio lead and senior national conservator (decorative arts) at the National Trust’s Royal Oak Foundation Conservation Studio, in Knole, Sevenoaks, Kent.

Visit the Royal Oak Foundation Conservation Studio website.