Actual laser removal of black soiling crust from siliceous sandstone by high pulse repetition rate equipment: effects on surface morphology

M A Iglesias-Campos,J L Prada-Pérez

doi:10.3989/mc.2016.02215

Abstract

This research project studies the role of pulse repetition rate in laser removal of black soiling crust from siliceous sandstone, and specifically, how laser fluence correlates with high pulse repetition rates in cleaning practice. The aim is to define practical cleaning processes and determine simple techniques for evaluation based on end-users’ perspective (restorers). Spot and surface tests were made using a Q-switched Nd:YAG laser system with a wide range of pulse repetition rates (5–200 Hz), systematically analysed and compared by macrophotography, portable microscope, stereomicroscope with 3D visualizing and area roughness measurements, SEM imaging and spectrophotometry. The results allow the conclusion that for operation under high pulse repetition rates the average of total energy applied per spot on a treated surface should be attendant upon fluence values in order to provide a systematic and accurate description of an actual laser cleaning intervention.

Highlights

The development of laser cleaning from its beginnings, and mainly since the late 1990s, has led to its frequent use on several architectural heritage projects
Through different spot and surface tests, this paper studies how laser fluence correlates with high frequency rates and average of total energy applied per spot on the basis of existing practices
Fluence indicates that results do not reflect pulse repetition rate and that the average of total energy per spot is clearly related to repetition rate and treatment time (Table 1)

Summary

Introduction

The development of laser cleaning from its beginnings, and mainly since the late 1990s, has led to its frequent use on several architectural heritage projects. The general standards or recommendations referred to in building-cleaning literature may include this technique [13,14,15], but when it is mentioned only general criteria are described. The main parameters controlling the technique are wavelength, pulse duration (characteristics of the equipment which the restorer cannot normally modify), fluence or energy density, repetition rate and whether the treatment is performed in wet or dry conditions (parameters adjustable by the restorer). Most of the parameters described focus on fluence, often accompanied by technical data of laser equipment which restricts knowledge of planning, treatment performance and extrapolation of data for similar situations

Methods

Results

Conclusion