Evaluation of Detachment Between Layers of Ancient Plaster Renderings: Comparison Between the Traditional Technique and a New and Innovative Automated Procedure Called PICUS

During a diagnostic campaign, most cultural heritage properties can be considered structurally as a multilayered system. The causes and phenomenology of degradation may involve the surface alone or the entire section of an ancient artifact. This includes the support, one or more preparatory layer, and the surface. An important section of diagnostics is dedicated to assessing the presence, position, and extension of separation between these layers. The results of these examinations are translated into graphic documentation of discontinuities to achieve conservative diagnosis and outline a project of maintenance/intervention. From 2018 to the present, research has been underway for the development of an acoustic–electronic procedure for the qualitative analysis of the presence of detachment and debonding in ancient artifacts. This procedure and the probe that was expressly developed in this regard are called the “PICUS” (the Latin word for “woodpecker”) system. PICUS is an automatic system for detecting and measuring detachments and nonvisible defects in general. The experimental apparatus consists of a probe made of an electromechanical percussion element that gently taps the surface producing a sound, a force sensor purposely developed to measure the impact force, and a microphone, all connected to an Arduino‐like low-cost board, to record and elaborate sounds and the force sensor signal. The probe XY position on the scene is recognized using an infrared camera system. The PICUS system has been validated by traditional vibro-acoustic measures. So far, it has been tested on various models and materials and ancient techniques, from glazed tile covers to mosaics to wood panels.This chapter is divided into four paragraphs. First, we describe the phenomenon of detachment of ancient renderings according to the lexicon in the field of conservation, normalized to define the forms of alteration. Here, we narrow the field to the detachments that occur between different layers in rendering plasters or mosaic bedding that employ lime as a binder. Then, we describe, from an acoustic point of view, the nondestructive testing (NDT) procedure in use by conservators and restorers for detachment diagnosis, according to the well-established procedure of tapping and listening. Other NDT techniques that can be successfully conducted in situ are briefly exposed. The complete PICUS system, hardware, software, and models for qualitative analysis of in situ detachments are then presented. Finally, we compare the results of two diagnostic techniques: the tapping technique traditionally used by conservators and restorers and the PICUS cross-correlation data processing algorithm. The maps obtained are compared, providing information on the effectiveness of the treatment and confirming the validity of the proposed method.