Proceedings e report - Firenze University Press

DEFIBRING OF HISTORICAL ROOF BEAM CAUSED BY AMMONIUM BASED FIRE RETARDANTS
Degree of damage is also influenced by thermal exposure during pretreatment, treatment and posttreatment
processing and in-service exposure, hence, by the combination of high temperature,
humidity and type and concentration of the present chemical.
Decrease of pH value significantly occurs during increased temperature (66°C) and may be regulated
by addition of pH buffers.
In her work, Lebow [7] tested the influence of the composition of the fire retardant based on
ammonium dihydrogen phosphate on the pH value of plywood. Based on her results she hypothesises
that ammonium dihydrogen phosphate converts into phosphoric acid at increased temperatures.
owever, signs of wood damage similar to those occurring after treatment with fire retardants based on
ammonium salts were also detected in the wood roof constructions near Sydney in Australia [8]. These
roof construction beams were destroyed due to the effect of sea salt. Wood defibring, however,
occurred here mainly on the bottom side of the beams and pH value of wood surface was within the
normal range but wood moisture content was very high, often more than 35 %. The ash analysis
showed high contents of sodium and magnesium cations. Examination of the wood under the electron
microscope uncovered the damage of middle lamella as well as the secondary cell walls (mechanical
damage and erosion). Wood degradation caused by wood-destroying fungi or bacteria was observed
very rarely.
2. Experimental part
The results presented in this paper were obtained based on analysis of wood samples taken from
damaged roof beams of the Český Krumlov Castle (Czech Republic), and of a house in street U
Půjčovny, Prague, which had presumably never been treated with any preservation procedure and
damage roof beams of the Old Royal Palace (Prague Castle), which had undergone a preservation
procedure, as mentioned above. Wood samples from the roof beam surface comprised loose fibres
(defibred wood) as well as wood chips split off from the surface by using a surgical knife.
Changes in the wood structure were examined by optical and electron microscopy. Chemical changes
(wood corrosion) occurring in the wood structure were studied by FTIR spectrometer connected to a
microscope. The reflectance technique was applied. The spectra were measure over the region of
4000-600 cm -1 at a resolution of 4 cm -1 applying 256 scans, and were normalized with respect o the
absorption band at 1032 cm -1 . Infrared spectroscopy was applied to samples of damaged wood (fibres
and chips) following their leaching in water.
The wood pH value was measured by the aqueous leachate method. The leachate was prepared from
0.5 g of sample (fibres, chips) in 50 ml of distilled water (previously boiled and cooled down). The
sample was allowed to leach for 1 hour with occasional stirring, the leachate was filtered, and its pH
value was measured with a combined pH electrode. The pH values of ammonium sulphate, ammonium
hydrogen phosphate, and ammonium dihydrogen phosphate solutions in distilled water at different
concentrations were measured as well. Each final value was obtained from triplicate measurement.
Furthermore, there were studied mechanical properties of the defibred wood surface layers in the
connection with the reference samples made of deeper (undamaged) wood layers that were extracted
from the damaged roof frame of the house in Prague, street U půjčovny. The observed properties were
compression strength and modulus of elasticity along the fibres as per Czech standard ČSN 49 0111
(size of samples 20×20×30 mm), tensile strength along the fibres (microsamples of trianglular cut
5×5×7,5 mm) and wood hardness measured by Janka method (Czech standard ČSN 490136) and by
using a Pilodyn 6J Forest resistance indentor.
Climatic conditions in the attic of the Old Royal Palace of the Prague Castle were monitored with
sensors providing temperature and relative humidity records.
3. Result and discussion
The surface of the damage beams are covered by fibres of different size and colour shade. In some cases,
the wood surface freed from the layer of released fibres is brown-red colour. Separation of wood fibres
(defibring) in the transverse direction can be seen when observing the damaged wood (chips and fibres)
under an optical microscope. In chip samples, the damage is associated with the formation of cracks and
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