Effects of hydrophobic treatments of stone on pore water studied by
continuous distribution analysis of NMR relaxation times
L. Appolonia
a
, G.C. Borgia
c
, V. Bortolotti
c
, R.J.S. Brown
c
, P. Fantazzini
d,
*, G. Rezzaro
b
a
Safeguarding Cultural Heritage Department, Piazza Narbonne 3, 11100 Aosta, Italy
b
CNR Laboratory, Safeguarding Culture Heritage Department, Via Grand Eyvia 57, 11100 Aosta, Italy
c
Dept. ICMA, University of Bologna, Viale Risorgimento 2, 40136, Bologna, Italy
d
Department of Physics, University of Bologna, Viale Berti Pichat 6/2, 40127, Bologna, Italy
Abstract
The effects of protective hydrophobic products applied to porous media such as stone or mortar vary greatly with the product, the porous
medium, and the mode of application. Nuclear Magnetic Resonance (NMR) measurements on fluids in the pore spaces of both treated and
untreated samples can give information on the contact of the fluid with the internal surfaces, which is affected by all the above factors.
Continuous distributions of relaxation times T
1
and T
2
of water in the pores of both synthetic and natural porous media were obtained before
and after hydrophobic treatment. The synthetic porous media are ceramic filter materials characterized by narrow distributions of pore
dimensions and show that the treatment does not produce large changes in the relaxation times of the water. For three travertine samples
most of a long relaxation time component, presumably from the largest pores, remains after treatment, while the amplitude of an intermediate
component is greatly reduced. For three pudding-stone samples, treatment leads to a substantial loss from the long component and an even
greater loss from the intermediate component. © 2001 Elsevier Science Inc. All rights reserved.
Keywords: Cultural heritage; Safeguard; Hydrophobic treatment; Magnetic resonance; Relaxation times
1. Introduction
The distribution of applied hydrophobic materials within
a porous structure, such as stone or mortar, is of particular
interest in the field of techniques used to safeguard the
cultural heritage. Nuclear Magnetic Resonance (NMR) re-
laxation analysis of pore fluids can supply information con-
cerning the distribution of the filming agents on the surfaces
of a porous medium by comparing the distributions of the
relaxation times of the fluid confined there before and after
treatment. This study gives the preliminary results of a
research aimed at defining more suitable protocols for the
protection and consolidation of the stone materials of Aos-
ta’s Roman Theater. Some samples of materials, both syn-
thetic and natural, were analyzed by traditional methods and
also by continuous distribution analysis of relaxation times
T
1
and T
2
of
1
H of water, both before and after a hydro-
phobic treatment with a silicone resin. The treatment and
also the saturation of the samples with water for the NMR
measurements before and after treatment were carried out
on evacuated samples. This does not reproduce normal
treatment conditions, but it offers the advantage of a simpler
interpretation of the variations in behavior of the relaxation
in the water before and after treatment, reducing the number
of parameters to be controlled to a minimum, with only a
single fluid phase in the pore system.
2. Materials and methods
The natural samples, 3 travertine (TR) and 3 pudding-
stone (PU), are cylinders 1.9 cm in diameter and 2 cm in
height. The two artificial samples are ceramic filter ma-
terial with homogeneous pore spaces (1.5 and 3.0 m
diameter) and are cylinders 2 cm in diameter and 1 cm in
height.
Sample preparation and control measurements are di-
vided into three phases. Phase One is the standard measure-
ment of water absorption and color change, following NOR-
MAL [1] procedures in use by the cultural heritage sector.
First, the sample is brought to its dry weight in an oven in
air at 60°C, and then the color is measured by means of a
* Corresponding author. Tel.: +39-051-2095119.
E-mail address: fantazzini@df.unibo.it (P. Fantazzini).
Magnetic Resonance Imaging 19 (2001) 509 –512
0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved.
PII: S0730-725X(01)00280-6