THE INFLUENCE OF OSMOTIC PRESSURE ON POULTICING TREATMENTS Leo Pel, 1 Victoria Voronina 1 and Alion Heritage 2 1 Tranport in Permeable Media, Department of Applied Phyic, Eindhoven Univerity of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherland 2 International Centre for the Study of the Preervation and Retoration of Cultural Property (ICCROM), Via di San Michele 13, 00153 Rome, Italy Abtract The crytallization of alt i widely recognized a one of the mot ignificant caue of irreverible damage to many cultural object uch a wall painting, tone culpture, and hitoric building. The removal of alt from thee object i however difficult and often poultice are ued. In thee method a wet poultice i applied to the urface of the ubtrate to be treated and i kept in place for ome period of time before being removed. Many tudie up to now on poulticing have focued on the alt and moiture tranport olely in term of advection and diffuion. The objective of thi tudy i to demontrate the potential contribution of omotic preure to alt extraction during poulticing treatment. To thi end, we have conducted a erie of experiment where we have meaured the moiture tranport during poulticing for ome well-defined material. Here we have ued Nuclear Magnetic Reonance (NMR) to meaure non-detructively the moiture tranport during thee experiment. Thi tudy how that omotic preure can exert a ignificant influence on alt extraction by poulticing method during drying. Importantly, a alt i tranported from the ubtrate and into the poultice, thi reult in a build-up of omotic preure within the poultice decreaing the effective pore-ize of the poultice. Therefore, the build-up of omotic preure enhance the alt extraction and thu increae the efficiency of the poulticing treatment. Keyword: poulticing, omotic preure, NMR 1. Introduction The crytallization of alt in porou media i widely recognized a one of the primary caue of irreverible damage to many cultural object uch a wall painting, culpture, hitoric building, and other artwork (Arnold and Zehnder 1991; Goudie and Vile 1997). Moreover, contemporary building and civil contruction alo uffer from alt-related deterioration procee. Salt crytallization can therefore be regarded a a common deterioration problem with ignificant cultural and economic implication. Poulticing i a common method ued in conervation to reduce the alt content of the affected object (Verge-Belmin and Siedel 2005). The methodology of application i relatively imple: the wet poultice i applied to the urface of the ubtrate to be treated and i kept in place for ome period of time before being removed. The dealination treatment by poultice include two main phae. The firt i the wetting phae: water i tranported from the poultice into the wall where it tart to diolve the alt. Water may penetrate the ubtrate due to water vapor tranport by mean of water vapor 1
diffuion or due to water capillary uction. The econd phae i the alt extraction. The diolved alt ion travel in the form of an aqueou aline olution from the ubtrate into the poultice. Thi alt migration can be the reult of two different procee. The firt i generated by the exitence of a concentration gradient between the ubtrate and the poultice. In thi cae the alt ion diffue through the olution. The econd i realized by the capillary water flow from the ubtrate to the poultice (generally due to drying) and i accompanied by ion advection within the olution. The preent reearch focue on alt extraction by drying poultice, during which advection i the main mechanim for alt removal. In the cae of advection baed alt extraction the efficiency of alt extraction i trongly dependent on the relative pore-ize range of the ubtrate and the poultice (Pel et al. 2010). Thi extraction proce i however potentially coniderably fater than diffuion baed method. Thi can to ome extent be achieved through the incluion of clay mineral (uch a kaolin) in poultice mixture (Lubelli and van Hee 2010) Up to now mot tudie on poulticing have focued on the alt and moiture tranport olely in term of advection and diffuion. However in alt extraction, a there are alt gradient preent o too will there be omotic preure gradient, which could have an influence. The aim of the work wa to invetigate the influence of omotic preure on poulticing. Firt, the fundamental apect of capillary tranport and omotic preure relating to a combination of two porou material drying will be dicued. Then, the NMR method and etup for meauring non-detructively the moiture ditribution, during the experiment on the influence of the omotic preure in poulticing drying i explained in Section 3. Finally, the reult of the experiment howing the influence of the omotic preure on the dealination proce will be dicued. 2. Theory The term 'advection' refer to the tranport of ma by a moving medium. In the cae of alt extraction by advection, the diolved ion can be tranported by the moiture flow from ubtrate into the poultice. Advection i generally more rapid than diffuion, and o dealination treatment baed on advection can be much fater. However, in order for advection from the ubtrate into the poultice to take place, certain requirement regarding the pore ize ditribution of the poultice and of the ubtrate need to be fulfilled; in particular the poultice mut contain a ufficient quantity of pore that are maller than the majority of thoe in the ubtrate (Pel et al. 2010). 2.1 Influence of the capillary preure In general the driving force for the tranport of water by advection i drying. During drying, the larget pore will empty firt, where the capillary preure (P c ) i lowet, a can be een from the following equation: 2γ Pc co( ) (1) r m 2
In thi equation r m i a pore radiu that dicriminate between the pore filled with water (r < r m ) and the empty pore (r > r m ), γ [Nm -1 ] i the urface tenion of the liquid/vapor interface and φ i the contact angle between the liquid/air and liquid/olid interface. In mot porou material the pore are not uniform, and therefore there i a pore ize ditribution. Hence, in thi cae the overall macrocopic capillary preure ψ c of the material i a function of it pore ize ditribution. For any moiture content θ [m 3 m -3 ], there will be a critical pore radiu, r m, correponding to a certain capillary preure that dicriminate between the pore filled with water and the empty pore. Hence, the macrocopic capillary preure ψ c i a function of the moiture content θ, which can be decribed thu: c c () (2) If we aume a perfect hydraulic contact at the poultice/ubtrate interface, the capillary preure will be continuou at thi interface, i.e.: p (3) p where ψ p i the capillary preure of the poultice, ψ the capillary preure of the ubtrate, and θ p and θ the moiture content of the poultice and ubtrate at the interface. Hence, due to difference in the poroity and pore ize ditribution between the two material there will be a jump in moiture content acro the interface. The relationhip between the moiture content either ide of the interface can be decribed thu: p 1 p f Hence, in general there will be a jump in moiture content acro an interface (4) 3
Figure 1. The pore ize ditribution a obtained by mercury intruion poroimetry for Migne limetone and Bentheimer andtone reflecting a fine and a coare porou material. A an example reflecting a poultice/ubtrate combination we can look at the drying of a combination of two material, i.e., Bentheimer, which i a material with coare pore, and Migne limetone, which i a material with fine pore. The pore ize ditribution a meaured by MIP for thee material i given in Figure 1. In Figure 2 the meaured moiture profile are given for the water aturated fine/coare material combination. A can be een the meaured moiture profile reflect the pore ize ditribution. In both cae the material with the larget pore drie firt, i.e. the Bentheimer andtone. Thee reult conform to the general idea that in order to extract alt by advection uing drying poulticing method, the poultice hould have pore maller than that of the ubtrate (Pel et al. 2010, Sawdy et al. 2010). Figure 2. The moiture profile a meaured during drying at variou time for a combination of a coare and fine porou material. The ample are dried at the left ide where a the right ide i ealed off. A a fine porou material, Migne limetone i ued, and a a coare porou material, Bentheimer andtone i ued. 2.2 Influence of the omotic preure When the ubtrate/poultice ytem contain a aline olution there will be an additional contribution to the macrocopic capillary preure due to the omotic preure for each material, i.e.: and the omotic preure, o, i given by: c, (5) RT o ln( a w ), (6) V w 4 o
where R i the univeral ga contant, T the abolute temperature and a w the water activity (for pure water a w =1 and hence the omotic preure i zero). When the well-known Pitzer' activity coefficient model i applied the omotic preure can be calculated a (ee e.g. Englezo and Bihnoin 1988): 3/ 2 2 m n 2 2 m n o RTm l 1 z z A1 ml A2 ml 2 (7) where 1/ 2 A I A1 (8) 1/ 2 11.2I A 1/ 2 2 0 1 exp 2I (9) A Φ i the Debye-Huckel coefficient, m l the molality of the olution, z+ and z- are ion charge, I the ionic trength of the olution, ν m and ν n are the number of mole of ion produced by one mole of the electrolyte (i.e., 2 for NaCl) and 0, 1, and 2 are the given parameter for Pitzer' activity coefficient model. The omotic preure of a NaCl olution i given in Figure 3 a an example a calculated uing the Pitzer model. A can be een the omotic preure reache almot 350 bar for a aturated odium chloride olution. Hence, due to the preence of oluble alt in a porou material, it effective pore ize (i.e., the equivalent pore ize for a water aturated ytem) will decreae a the total macrocopic capillary preure i increaed by the omotic preure of the alt. Hence, due to the alt aborbed in the poultice it effective pore ize will decreae. 5
Figure 3. The omotic preure of NaCl olution a a function of the concentration a calculated from the Pitzer model. 3. NMR In thi tudy we have ued Nuclear Magnetic Reonance (NMR) to meaure nondetructive and quantitative the moiture in the ample while drying. NMR i baed on the principle that in a magnetic field, nuclei have a pecific reonance frequency and can be excited by a radio frequency field. The reonance frequency f (Hz) depend linearly on the magnitude of the magnetic field: f 2 B 0 (10) Where γ/2π (HzT -1 ) i the gyromagnetic ratio, B 0 (T) i the main magnetic field. For 1 H γ/2π i 42.58 MHzT -1 and 23 Na i 11.26 MHzT -1. Therefore, by uing a pecific frequency the method can be made enitive to a particular type of nucleu, in thi cae either hydrogen or odium. The ignal intenity S of a pin echo a ued in the experiment i given by: T r T e S 1 exp exp (11) T1 T2 where S i ignal intenity, ρ i the denity of the hydrogen nuclei, T r and T 1 are the repetition time of the pule equence and pin-lattice relaxation time, T e and T 2 are the pin echo time and pin-pin relaxation time. To meaure the maximum ignal, i.e. from all pore ize, T e hould be a hort a poible a T 1 and T 2 are proportional to the pore ize. For the preented experiment, a home-built NMR canner with a tatic magnetic field of 0.78 T and gradient up to 0.3 T/m i ued (Petkovic et al. 2007). To perform quantitative meaurement a Faraday hield i placed between the coil and the ample. 6
Figure 4. A chematic diagram of the NMR etup for drying experiment. The Teflon holder with the ample and the tandard i moved in the vertical direction by mean of a tep motor. The experimental et-up i given in Figure 4. The ample, which ha a cylindrical hape with a diameter of 20 mm and a length up to of 80 mm, i moved vertically through the magnet with the help of a tep motor. It i ealed at all ide, except for the top over which air with a relative humidity of 5% i blown. In thi way, a onedimenional drying proce i created. After each meaurement the ample i moved in the vertical direction by the tep motor. The meaurement time for the moiture content at one poition i in the order of 1 min. Thi procedure i repeated until a complete moiture profile ha been meaured. A time tamp i given to each meaurement point. 4. Omotic preure influence In order to how the effect of purely the omotic preure we have conducted an experiment where dealinate a Bentheimer with a poultice with the ame pore ize ditribution. If we would only take the capillary preure into account we would not expect any advection and therefore no dealination effect. In thi experiment we have dried a Bentheimer aturated with 5M NaCl olution on top of a 2 M NaCl olution aturated Bentheimer. In thi cae due to the higher alt concentration the effective pore ize of the Bentheimer with 5 M NaCl olution will be maller becaue of the omotic preure. The reulting meaured moiture profile during drying are given in Figure 5. A can be een indeed the Bentheimer with 5 M NaCl olution dried more lowly indicating that it effective pore ize i maller due to the omotic preure. 7
Figure 5. The meaured water profile in a Bentheimer/Bentheimer ytem for everal time during drying. Initially the Bentheimer on the top wa aturated with 5M NaCl olution and the ubtrate Bentheimer wa aturated with 2M NaCl olution. The correponding moiture content at the interface for thi drying experiment i plotted in Figure 6. Figure 6: The water aturation of the 2M NaCl olution aturated Bentheimer at the interface a a function of water aturation of the 5M NaCl olution aturated Bentheimer at the interface (ee alo Figure 5). The olid line repreent the relation when both material are water aturated 8
wherea the dahed line repreent the relation a determined from the capillary preure curve and the calculated omotic preure. A can be een, there i a clear deviation from the curve a expected from pure capillary effect. After ome time, a deviation can be een from the relationhip a predicted on bai of the calculated omotic preure. Thi i due to the alt tranport in the Bentheimer aturated with 2M NaCl olution toward the interface, a a reult of which the concentration difference decreaed at the interface and the omotic preure will change. 5. Concluion Thee drying experiment demontrate the effect that omotic preure ha on alt and moiture tranport within porou material. A aline olution in a porou material will exert an omotic preure that will to reduce the effective pore ize of the material. Thi tudy how that the contribution of omotic preure can exert a ignificant influence on alt extraction uing poultice during drying. Importantly, a alt i tranported from the ubtrate and into the poultice, thi reult in a build-up of omotic preure within the poultice, thereby enhancing the extraction. Thee finding have potential practical implication for the optimiation of poulticing treatment. While the pore ize requirement for advection to take place at the tart of the proce remain, thee contraint need not be quite o evere. They are gradually overcome by the build-up of an omotic preure due to the on-going migration of alt from the ubtrate to the poultice. A a reult, the longer a poultice tay in contact with the ubtrate, the more it will accumulate alt, and thereby the omotic preure i increaed and it effective pore ize will become maller enhancing the dealination proce. Reference Arnold, A. and Zehnder, K. 1991. Monitoring wall painting affected by oluble alt. In The Conervation of Wall Painting, Cather, S. (ed.) 103-136. Lo Angele: The Getty Conervation Intitute. Englezo, P. and Bihnoin P.R. 1988. Prediction of ga hydrate formation condition in aqueou electrolyte olution. AIChE Journal, 34: 1718-1721. Goudie, A. and Vile, H. 1997. Salt weathering hazard. Wiley: Chicheter Lubelli, B. and van Hee, R.P.J. 2010. Dealination of maonry tructure: Fine tuning of pore ize ditribution of poultice to ubtrate propertie. Journal of Cultural Heritage, 11: 10-18. Pel, L. Sawdy, A. and Voronina, V. 2010. Phyical principle and efficiency of alt extraction by poulticing. Journal of Cultural Heritage, 11: 59-67. Petkovic, J. Huinink, H.P. Pel, L. Kopinga, K. and van Hee, R.J.P. 2007. Salt tranport in plater/ubtrate layer. Material and tructure, 40: 475-490. Sawdy, A. Lubelli, B. Voronina, V. Funke, F. and Pel, L. 2010. Optimiing the extraction of oluble alt from porou material by poultice. Studie in Conervation, 55: 26-40. Verge-Belmin, V. and Siedel, H. 2005. Dealination of maonrie and monumental culpture by poulticing: A review. Retoration of Building and Monument, 11: 1-18. 9