Proceedings e report - Firenze University Press

PREDICTION OF LINEAR DIMENSIONAL CHANGE OF UNRESTRICTED WOOD
AT DIFFERENT LEVELS OF EQUILIBRIUM MOISTURE CONTENT
Michael Formosa 1,2 Martin Musumeci 2
1 Conservation Division, Heritage Malta, Malta
2 Institute of Conservation and Management of Cultural Heritage, University of Malta, Malta
Abstract
A computer routine to predict variations in linear dimensions and warping of wood was designed and
tested. This routine was applied on both experimental data obtained from laboratory-prepared wood
samples as well as other data found in the literature. The two outcomes were compared and contrasted.
Predictions resulting from the computer procedure resulted to be reliable only for wood which is
straight grained and free from physical constraints and natural defects
1. Aim
The aim of the computer analysis exercise was to predict linear dimensional changes as well as any
possible warping (cupping) of unrestricted wood.
2. Method
A computer routine was devised by using two computer software packages, namely Microsoft Excel
and AutoCAD 2000. Data found in the relevant literature was used in order to carry out the necessary
workings. The four stages of the computer analysis routine consist of:
(i) the end grain photography and line drawing on AutoCAD;
(ii) the merging of data into a Microsoft Excel spreadsheet;
(ii) the interpretation of data and the relevant calculations; and
(iv) the re-construction of the panel end grain, considering the predicted contraction/expansion and
warping using AutoCAD.
2.1. End grain photography and line drawing by AutoCAD
The first step involved capturing an image of the end view of the panel. In this study, both direct
scanning and digital photography were applied. One has to consider that photography has an element
of inaccuracy due to perspective illusion.
The picture was transferred into AutoCAD and the outline traced. The drawing was then divided into a
number of quadrilateral sections, as in the case of ABCD shown in Fig. 1. In areas where the radius of
curvature of the annual rings was quite small or complex, smaller quadrilateral sections were
constructed. The diagonals in each section were then marked (as AD and BC in Fig. 1) and the
orientation of the grain was marked (as EF in Fig. 1). All dimensions (of lines AB, CD, AC, BD, AD
and BC) and angles of orientation (0º to 90º) were measured using AutoCAD.
All the following data was then fed into a Microsoft Excel spreadsheet:
• initial temperature (T);
• initial relative humidity (RH);
• final T;
• final RH;
• literature value for longitudinal shrinkage (left out if length of panel was not being considered)
and when data was not available, a value of 0.1% was assumed; [1]
• literature value for tangential shrinkage of the given type of wood (left out if the width and the
thickness of the panel were not being considered);
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press