All that glitters is not gold, or is it?

Photo: Gilded mock-ups

All that glitters is not gold, or is it?
Photo: Painting with gilding. (c) INP/Angèle Dequier

Gilding is a form of polychromy in which a very thin sheet of gold is placed on top of a surface and attached using different methods. It creates the visual effect that the surface is indeed made of gold. This was extremely common in the Middle Ages, and it is one of the most important components of medieval polychromy. However, it has been used throughout millennia and all around the globe. Some examples of polychromy are Egyptian sarcophagi or altarpieces in gothic churches. The appearance of gold was meant to symbolise the wealth of a person and the magnificence and splendor of the “House of God”.

Many paintings and religious icons are also gilded. However, due to exposure to the elements, bad storage and conservation conditions, and ageing in general, the original appearance of the gilded surface is lost. For this reason, it is important to study the appearance of gilding and how its appearance changes due to different conservation and restoration procedures. Early-Stage Researcher (ESR) 7, Yoko Arteaga, is working on characterising the appearance of gilding to guide conservators’ restoration treatments. In collaboration with the National Institute of Cultural Heritage (INP) in Paris, she is working with a conservation student who has made different gilding mock-ups replicating techniques from the Middle Ages.

Object appearance

The appearance of an object is linked to three main factors: its shape, the material it is made from and illumination it is under. All these three components are then perceived by an observer as the appearance of said object. Moreover, the appearance of an object can be described by its colour, translucency, gloss, and texture. Devices such as cameras or spectrophotometers can record aspects of appearance however, there is no machine capable of capturing an object’s global appearance on its own.

Photo: Yoko preparing the mock-ups

Light absorbing pigments on diffuse materials such as paper can be easily described using single geometry measurements which have a good correlation to their perceived appearance. However, glossy materials with high specular reflection, such as gilding, are more complex and their appearance varies when looked at and illuminated from different angles.

An important way to describe how an object looks from a given angle of observation at different illumination conditions is its Bi-directional Reflectance Distribution Function (BRDF). This function is widely used when rendering the colour and gloss of materials. It is specific to an objects’ surface properties and takes as an input the angle of observation and angle of illumination and returns as output the colour of the object under these conditions.

Reflectance measurements instruments consist of a light source to uniformly illuminate the object and a detector such as a camera or sensor to measure the amount of light reflected. To obtain very accurate measurements of the BRDF, gonio-spectrophotometers are used. However, these are very costly, slow, and mainly available in research institutions or dedicated metrology laboratories. Thus, there is a real need, specially in the cultural heritage field, to develop more accessible methods and devices which can be used in museums, restoration workshops or archiving facilities.

Goniometric reflectance measurements

As part of her PhD thesis, Yoko has been working on developing such a technique which can provide information about the appearance of an object’s surface when viewed and illuminated from different directions in a simple way and using widely available equipment. Using just a DSLR camera, a light source and a tilting platform, the sparkle effect of bronze patinas was characterised. This work has been presented in the 29th Colour and Imaging Conference and the paper has been published in the conference proceedings.

A simple DSLR camera and a light source such as a ash are positioned at fixed positions making a 45- degree angle between them. The sample is placed on a tilting platform and at each angle of tilting a picture of the sample is taken. Then the images are processed to obtain information of how the reflectance of the surface changes as it is illuminated and viewed from different directions.

The samples’ surface topography was also characterised using a white-light prolometer. This gives a 3D map of the peaks and valleys present in the surface structure at a microscopic scale and helps characterising the appearance of the objects. Smooth surfaces with a low roughness present high specular reflection, making them glossier, whereas rougher surfaces with many micro-facets oriented at different directions give a diffuse reflection and matt appearance.

Photo: Yoko working with the robotic arm

Secondment work Yoko has recently completed a three-months secondment at NTNU where she has worked on acquiring appearance measurements of gilding samples using different techniques. More specifically, she has been using an image-based set-up with an automated illumination thanks to a robot arm. This allows the light to be incident on the surface from almost any direction and the reflection is captured by a camera. With this information, the BRDF of the surface can be estimated. She hopes that these results will complement and validate the results from her own work. She has also expanded the range of gilding samples to study, as she has gained access to mock-ups fabricated imitating a traditional method of “imitation” gilding commonly found in medieval sculptures from Northern Europe. This gilding is not made with gold leaves, but instead, a silver sheet is attached to the surface and a yellow resin is applied on top, giving the striking appearance of gold. This investigation in the appearance of different types of gilding can help conservators further understand the interesting appearance properties of these materials.