Proceedings e report - Firenze University Press

DEFIBRING OF HISTORICAL ROOF BEAM CAUSED BY AMMONIUM
SULPHATE AND AMMONIUM PHOSPHATES BASED FIRE RETARDANTS
Irena Kučerová 1 *, Martina Ohlídalová 2 , Jiří Frankl 3 , Michal Kloiber 3 , Alena Michalcová 4
1 Department of Chemical Technology of Monument Conservation, Institute of Chemical Technology Prague,
Czech Republic
2 Central Laboratory, Institute of Chemical Technology Prague, Czech Republic
3 Institute of Theoretical and Applied Mechanics (ITAM) of Academy of Sciences of Czech Republic
4 Department of Metals and Corrosion Engineering, Institute of Chemical Technology Prague, Czech Republic
Abstract
Some chemicals, which are ingredients present in preservatives, can induce chemical reactions that cause
damage to wood polymer. Among such chemicals are ammonium sulphate and ammonium phosphates in fire
retardants, which cause damage to historical wood surface, called defibring. Such damaged has been reported to
exhibit enhanced acidity and moisture content. Defibred wood was studied by electron microscopy and infrared
spectroscopy, by pH value measurement and termogravimetry. Measurements of climatic conditions in the attics
and mechanical properties of defibred wood were carried out. It is concluded that the fibres on the wood surface
consist mainly of cellulose, which is occasionally partly damaged, and that, contrary to general belief, the
observed increase in wood acidity is not due to the previous application of fire retardants and subsequent acid
hydrolysis; instead, this effect seems to be due to oxidation of the structural components of wood. Defibring
affects only mechanical properties of the wood surface layers.
1. Introduction
Some chemicals that are present in preservatives can induce chemical reactions, which cause damage to wood
polymers. Among such chemicals are ammonium sulphate and ammonium phosphates that serve as
components of fire retardants. Many roof beams in historical buildings in the Czech Republic have been
repeatedly treated with these fire retardants. This treatment brought about damage to the historical wood
surface consisting in a release of loose wood fibres, named as “defibring”. For instance, the roof beams of the
Old Royal Palace within the Prague Castle (Czech Republic) are damaged by defibring. There were subject to
extensive investigation roughly in 1997-2001 [1-4]: The wood pH value was 4-5 in dependence on the
sampling point. Damaged wood was removed from the beam surface with a brush and the beams were treated
with a neutralizing solution containing calcium carbonate and boric acid. The composition of the solution was
modified so as to match the observed acidity of the wood. The neutralization process resulted in a pH value
increase roughly to 5-6. Subsequently the parts were treated with a boric acid based protective agent. As it
appears, however, this procedure is inefficient, and in a few years the process of defibring of the wood surface
will set in again.
Such damaged wood has been repeatedly reported to exhibit a higher acidity and increased moisture
content. The latter effect is attributed to the hygroscopic nature of the salts. The change in the wood
pH value is believed to be a result of salts dissociation due to high air humidity, resulting in the
formation of acid or alkaline solutions. Therefore it is assumed that the wood damage mechanism
consists in hydrolytic degradation or acid catalyzed dehydratation. The extent of this effect depends on
the salt concentration and on temperature. In addition, highly hygroscopic salts are assumed to exert a
mechanical effect arising from crystallization pressures. [4-7]
The effect of fire retardants based on ammonium sulphate and ammonium phosphates has been
examined extensively by USDA Forest Service, Forest Products Laboratory (e.g. by J. E. Winandy
and S. L. LeVan), particularly in relation to a decrease in the strength of plywood sheathings and
strandboards under conditions of increased temperature and humidity. Knowledge from the previous
studies is summarised by Winandy in his work [5], where he states the following:
Mechanism of thermal degradation in fire retardant treated plywood is acid hydrolysis.
* E-mail: Irena.kucerova@vscht.cz
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