Salt efflorescence in historic wooden buildings
The protocol here envisaged allowed performing a characterization of the efflorescence in the buildings under investigation. On site examinations by XRF led to the identification of a number of chemical elements in the efflorescence and ascertained the occurrence of inorganic salts not only over the wooden material but also within the peripheral wood layer. Laboratory assays of the plain efflorescence brought about supplementary details on the chemical composition by disclosing additional chemical elements and molecular species, and revealing the occurrence of double salts of the alum and Tutton’s compound types. Finally, chemical analysis of the recrystallized efflorescence provided an accurate assessment of the ionic composition of the specimen.
Depth profiling of representative ions (ammonium and sulfate in the present case) ascertained the occurrence of the salts within the peripheral layer down to about 6 cm. If these ions are missing from the efflorescence, other indicator ions can be selected and determined by suited methods such as ICP-OE or IC.
The efflorescence in Aspaas House consists of inorganic sulfate salts containing the following major cations: ammonium, aluminium, magnesium, zinc, potassium, sodium, and iron. Ammonium accounts for about half of total moles of cations.
In order to ascertain the origin of the efflorescence it would be of interest to look first at the content of inorganic components in wood. A compilation of nutrient content in gymnosperms wood (as mass % of dry matter) shows the following average figures for metal ions: calcium, 0.097 ± 0.101, potassium, 0.080 ± 0.120, and magnesium, 0.019 ± 0.012 [35]. Besides the nutrient elements (nitrogen, phosphorus, potassium, calcium, magnesium), iron, copper, zinc, sodium and manganese are present at the ppt concentration level in wood [21]. Consequently, the main components of the efflorescence cannot be of biogenic origin but arise rather from the products of a preservation treatment applied to the wooden material. Most likely, the treatment was applied to the building elements before they were re-assembled on the museum site at the beginning of the twentieth century. At that time, impregnation of museum wood artifacts with inorganic biocides and fire retardants was a well-established practice [5].
The most likely cause of efflorescence seems to be water leakage at a not-recorded date, probably due to roof damage. Leaking water penetrated the wood and dissolved the salts inside forming a salt solution contained within a thin layer at the surface. Subsequent water evaporation and salt crystallization occurred after stopping the water leakage.
According to data in Table 1, most of the efflorescence in Aspaas House consists of double sulfate salts of the alum and Tutton’s salt types. However, except for potassium alum and ammonium iron sulfates, which were available as industrial products at the moment of house reassembling, other double sulfates cannot originate from the salt blend applied to logs. More probably, the original salt mixture consisted mostly of simple sulfate salts that generated double salts thereafter, during the crystallization process. The chemical composition of the efflorescence suggests that the impregnation mixture had the following composition (inferred content is given in mass % for each salt): ammonium sulfate ((NH4)2SO4; 30), aluminium sulfate (Al2(SO4)3·18H2O; 15), magnesium sulfate (MgSO4·7H2O; 15), iron(II) sulfate (FeSO4·7H2O; 15), zinc sulfate (ZnSO4·7H2O; 10), sodium sulfate (Na2SO4·10H2O; 10), and potassium alum (KAl(SO4)2·12H2O; 5). When such a mixture is dissolved, the large content of ammonium ion renders the solution slightly acidic, thus preventing oxidation of iron (II) [36] as well as the hydrolysis of aluminium and other hydrated cations. Manganese and other low-concentration elements may originate from either mineral components of the wood or impurities in the preservative salt blend.
The composition of the efflorescence in the Tronshart House is less intricate. Its chemical composition suggests that it may contain a Tutton’s salt ((NH4)2Mg(SO4)2·6H2O) but also plain ammonium sulfate.
It was already stated that efflorescence is associated with salt crystallization from a solution contained in a porous substrate. In typical efflorescence, salt-containing solution is constantly supplied by capillary action to the evaporation front so that the front position remains unchanged. Conversely, if moisture reaches the salt-containing porous substrate either accidentally or by deliquescence, subsequent evaporation will involve only a limited amount of solution. Consequently, salt precipitation may start at the surface as efflorescence and continue inside the material as subflorescence. This is the case of the buildings investigated in this paper.
According to literature data, single salts or mixtures of only a few salts were commonly utilized in the past for wood conservation [5]. The complex composition of the salt formulation applied to the Aspaas House is therefore an intriguing exception. Certainly, this intricate formulation was intended to provide effective protection against both fire and biological degradation. The preservative mixture applied to the material in Tronshart House, which is of a less architectural importance, consisted only of ammonium and magnesium sulfates, probably for economy reasons.
The effectiveness of the treatment can be assessed in terms of toxicity to microorganisms, permanency, retention, and depth of penetration into the wood [37]. It is known that salts of alkali metals, magnesium, and zinc (especially sulfates and chlorides) were applied to wood artifacts for biocidal purpose until the advent of organic biocides [5, 7]. The salt blend applied to the Aspaas wooden material contains sulfates of all the above-mentioned metals. Among the above ions, zinc appears as the most efficient biocide and zinc chloride was widely used as a preservative at the beginning of the twentieth century [4]. At the same time, since magnesium and sodium sulfates were used in the past as fire retardants [5], it may be assumed that these salts were selected in order to offer double protection. However, the permanency of soluble salts is only satisfactory in the indoor where the material is not exposed to water. As expected, XRF data proved that the salts were washed out by precipitation from the outdoor areas. The retention (i.e., the amount of preservative that must be impregnated into a specific volume of wood) seems to be satisfactory as far as the peripheral layer is concerned, but the depth of penetration is rather low.
Fire protection is provided by several components of the applied salt blend. Ammonium sulfate, which is the most abundant salt in the treatment mixture, is a well-known flame retardant [38, 39]. At elevated temperature, it decomposes and releases gaseous ammonia and sulfur oxides, which dilute the flammable gases produced by cellulose pyrolysis. At the same time, these gases blanket surfaces and limit the access of atmospheric oxygen to the fiery zone thus reducing the rate of charring [40]. Similar protection is provided by water vapor originating from crystallization water that is abundant in the mixture components.
The above fire protection mechanisms are of physical nature. But flame-retardation action is also provided by a chemical mechanism based on charring via dehydration of cellulose below the flaming temperature. As a result, the amounts of flammable tars and gases decrease and the amount of the much less combustible char increase [40]. Cellulose dehydration is catalyzed by dehydrating agents such as sulfuric acid that may form by high temperature decomposition of metal sulfates included in the treatment compound. Cellulose dehydration is also catalyzed by Lewis acids [40] and, among the ions present in the salt blend, the following Lewis acids can be listed in the decreasing order of the hardness parameter [41]: Na+ Mg2+ Fe2+ Zn2+ Al3+.
As far as the treatment technology is concerned, the above-mentioned depth of penetration suggests a treatment by prolonged immersion (steeping) [4]. Depending on timber condition and provenance, this method is able to secure typically a depth of 5–30 mm [42] which agrees with the penetration determined for the investigated material. In terms of costs and benefits, such an immersion treatment is very convenient [43].
In summary, the complex salt blend applied to the Aspaas House was devised to provide both fire and biological degradation protection. Both well-established empirical knowledge and current theoretical approaches prove that this salt mixture is very efficient as a flame retardant.
Besides the favorable protective effect, possible impairment of wood tissue due to salt preservatives cannot be ruled out. The salt deposit may undergo deliquescence if the relative humidity in air (RH) overcomes the deliquescence threshold, RH0. Alternating deliquescence-efflorescence cycles may occur and cause stress within the porous substratum [44]. Besides, moisture produced by deliquescence may stimulate the apparition of fungi. As a rule, for a blend of deliquescent substances, the RH0 will be lower than that of each component, which prompts deliquescence to occur at lower RH [45]. However, this effect is less marked in the presence of a common ion [46], as it occurs in the system here investigated in which sulfate is the single anion.
Another problem arising from moisture is the chemical degradation due to acidity produced by metal ion hydrolysis as it was documented in the case of both archaeological waterlogged wood, and fresh wood treated with potassium alum [47].
Wood material, exposed to salt contamination from environment, experienced degradation of the middle lamella between cells, which caused the remaining cells wall layers to separate. As a result, wood fibers are released and wet wood appears as if the surface cells have been pulped. This process was noticed in polar marine environment [48] as well in an abandoned saltpeter work site [49]. The degradation mechanism is not yet known but it was presumed that in the presence of moisture, the dissolved salts would cause a high pH. Consequently, a chemical attack on lignin would occur in a process that could be comparable to the process of alkali pulping of wood. However, this kind of degradation cannot occur in the buildings here investigated since the large content in ammonium prevents pH to shift to the alkaline range.
Making a full assessment of the health hazard of salt efflorescence is beyond the scope of this work. Actually, a rigorous assessment is impossible because no safety data are available for all double sulfate salts included in the efflorescence. However, as health hazard is due rather to the constituent ions than to the salt itself, some inference can be made from security data sheets of presumed components of the original salt mixture. Accordingly, these salts are not critically hazardous but it is advisable to avoid any kind of exposure to the efflorescence material. Although this material is not volatile, absorption in the human body may occur by inhalation of contaminated dust particles and ingestion via mucociliary clearance [50]. Considering the large specific surface of the efflorescence, this risk should not be ignored at all, particularly if cleaning by mechanical procedures is attempted.
Removal of the efflorescence would be beneficial for the aspect, and to reduce health hazards and risks of wood degradation. Some of the cleaning techniques reviewed in [7] appear suitable in this case. The simplest one is the mechanical procedure using vacuum cleaners fitted with HEPA filters. As the efflorescence shows poor adhesion to the wood, it could be removed in this way but the salts incorporated in the wood will persist. Removal of both efflorescence and incorporated salts could be achieved by the vacuum washing process [51, 52]. This approach is more costly and laborious but such a deep-cleaning treatment eliminates the risk of efflorescence return and wood degradation under the effect of incorporated salts.