Friday, 26 July 2013

A month in the making

With one thing or another I've let this slip recently, so this post will cover what I've been up to over the past month. It's been ferociously busy, but so much fun.

I recently did a huge amount of paint analysis. This included grinding and polishing paint samples, observing in polarised and UV light, taking photomicrographs, before compiling into  one image. Subsequent analysis included stain-tests to confirm presence of components such as proteins and carbonates.

Examining paint samples
Sample photomicrograph in visible and UV light

A lot of time was spent painstakingly dusting gold-leaf skewings and snagging the Trophy Gates from Hampton Court Palace, before they were re-installed at the palace entrance. This work took place both at our workshop and on site following installation, where the undercoat could be seen grinning through in bright daylight. While there is not a lot more that can be said about this, I've included a lot of photos to illustrate the final phases of their royal treatment.

Snagging and skewing

How many people does it take to
move a two tonne gate!
No time wasted moving in

Grinning undercoat
Palace and petal

Elements in stages


Over the past couple of weeks, I've also had an opportunity to work on the gypsum plaster panels from Pegasus House.

The low relief carved plaster panels were designed by the sculptor, Denis Dunlop, in the 1930s. They are formed from hessian-reinforced gypsum with an integral timber frame.

Historical evidence, supplemented by paint analysis, confirmed that the reveals have always been painted uniformly in an ivory colour. The panels, flooded by light through the window, use this position to cast measured shadows, to emphasise the relief and raise the pattern out of the plaster.

There are ten panels in total, charting the natural and powered history of flight. The stylised Art Deco scenes include dandelion clocks and maple helicopters spiralling in the wind, Icarus and his fateful flight to the sun, winged fish rising from the ocean to meet swooping gulls and soaring eagles, planets spinning in starry skies over sleeping towns, hot-air balloons floating dreamily along while zeppelins drone overhead, and biplanes gliding through billowing clouds over sweeping aerial townscapes.

Since my last post, the panels had been cleaned to a suitable level and the full extent of the damage could be ascertained. Aside from the extensive network of fractures and splits, the panels had suffered severe sulphate attack, leaving the entire surface pitted and scarred. In some areas nothing more than a thin, friable skim remained, through which the underlying hessian poked through.
Sulphate damage

Damaged, friable surface

As they were cast from gypsum, the panels are incredibly soft and so a number of different repair materials and techniques were tested. 

Fractures needed to be filled with a material strong enough to restore structural integrity to the panel, while also being as soft, if not softer, than the existing plaster. Other factors to be considered included water content of the filler, as gypsum is soluble in water, and flowability, as the material needed to be worked into the fractures to be effective. 

Plane detail - final
For re-creating missing detail and smoothing pitted flat areas, the plaster mix need a whole set of different properties, predominantly good workability, as sections were re-modeled by hand (and had originally been cast), effective adhesion to the substrate, relatively fast setting-times and hardness. (but not harder than the original!) PHEW!

A number of different mixes of calcium sulphate hemihydrate (gypsum), binders and adhesives were trialed to find the composition which ticked all our boxes.
Aerial view field system
(Thanks, TB!)

To begin with, fractures were filled to consolidate the panel. To achieve an effective fill, the scalpel blade was drawn through the crack and any dust or gypsum particles clogging it were removed by suction-at-source extraction. The crack was lightly moistened to maximise binding efficacy, and the filler was packed neatly into the void.

To repair surface scars, the area was cleaned, keyed, moistened and the filler was spread across the surface. For deeper pitting, it was necessary to build the material up in stages, so as to avoid shrinkage cracking.

Cast section for detail
Remodeling missing and irreparably damaged sections was one of the loveliest and most frustrating jobs I've ever done! Using sections cast from existing fibre-glass moulds as guides, areas were built-up freehand with modeling tools using an appropriate gypsum mix. 

Deciphering the pattern the first step, then new and old areas needed to be seamlessly linked in the same plane, lines needed to be crisp and true and form needed to be perfect. Barely perceptible nuances in the pattern, such as beveled edges and subtle peaks, coupled with the incredibly low-relief of the design made this all the more tricky. 

Sections which were so badly damaged that sufficient key to the substrate could not be achieved, were cast from the existing mould, the areas cut back and the new section inserted and surrounding areas filled.

Hot-air Balloon in stages
Wing detail - before and after

Thistle in stages

Gulls - before and after

One panel, featuring Icarus, was lost beyond repair, so a new piece had to be cast from the fibre-glass mould.

Recast Icarus
Now that all the panels have been repaired, they are being prepared for re-instatement at Pegasus House. More to follow on this.

Eagle panel in-situ
Eagle head detail

Eagle wing detail
Eagle final

(As an aside:
- There are some lovely interviews and anecdotes here charting the history of Pegasus House.

- I am very keen to find out more about the sculptor, Denis Dunlop; so far, I've only managed to find two other buildings he has contributed to - Assembly Hall, Lambeth and RIBA HQ. If anyone happens to know anything more about him, I would be grateful if you could get in touch.)

Croxley Great Barn
In between all of this, I also spent a few days at Croxley Great Barn, Rickmansworth, as part of the Society for the Protection of Ancient Buildings working party. The 14th century timber-trussed, flint-clunch-and-brick-(!)-walled threshing barn is believed to have been built as part of St Alban's Abbey. Given it's age, it is remarkably unchanged, and given it's abandonment, is in relatively good order. 

Over the course of the week, with guidance from the SPAB technical panel and guardians, volunteers carried out a huge amount of work, including a survey of the threshing floor, installation of temporary rain-water measures, repair of damaged plinth walls, removal of vegetation and consolidation of delaminated flint layers.

As ever with the SPAB, it was an excellent week and I will definitely be back next year! Furthermore, you may be interested in catching up with the SPAB Scholars and Fellows' conservation adventures.

Timber-clad north elevation

Leached mortar and flint losses creating massive voids and destabilising wall.

Slaving over the mortar mix.

Packing voids with 2:1 lime mortar with flint inclusions for added key.

Admiring some knapped flint.

Repointing and rebuilding the loose outer layer, having stabilised and tied the flint back into the core of the wall.

... A much happier looking wall!

This post has gone on far too long already, but should hopefully bring everything up to speed. A final other few bits and pieces I've been up to include lots of pH, moisture and salts profiling, a quick visit home to Cork, Ireland, attending the BAPCR conference, "The Picture So Far ... 50 Years of Painting Conservation", and generally melting in this cruel, cruel heat...