Eleventh Annual V. M. Goldschmidt Conference (2001) 3212.pdf

DAMAGE TO MONUMENTS FROM THE CRYSTALLIZATION OF MIRABILITE, THENARDITE AND HALITE: MECHANISMS, ENVIRONMENT, AND PREVENTIVE POSSIBILITIES. E. Doehne1, C. Selwitz2, D. Carson1, and A. de Tagle1, 1The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049, USA, TEL: (310) 440 6237, FAX: 310 440 7711, Email: [email protected], 23631 Surfwood road, Malibu, CA, 90265, USA.

Introduction: "In time, and with water, everything sulfate through columns of limestone. The limestones changes." ---Leonardo da Vinci were oolitic Monks Park stone where 90% of its pores The crystallization of soluble salts in porous building measured between 0.05-0.80 microns and Texas materials is a widespread weathering process that re- Creme, a fossilferous stone with 90% of its pore size sults in damage to important monuments and archaeo- distribution measuring between 0.5 and 3.0 microns. logical sites [1]. Salt weathering by thenardite (sodium In the absence of modifiers, sodium chloride passage sulfate) and mirabilite (sodium sulfate decahydrate) is through Monks Park limestone gave predominantly especially destructive, yet is still not fully understood subflorescence with mild edge erosion while sodium [2]. Halite (sodium chloride) in contrast, is one of the sulfate mainly effloresced and severely damaged the least damaging salts [3]. Previous work has also dem- stone column. With Texas Creme limestone, essen- onstrated the importance of airflow in salt weathering tially only efflorescence occurred with either salt and [4]. Here we present new data that help explain why there was little or no stone damage. sodium sulfate is so damaging and also show how Most crystallization inhibitors--used in industry to crystallization modifiers and changes in airflow can prevent scaling by promoting high supersaturation reduce salt damage in laboratory experiments. levels for calcium compounds--plugged the columns or Damage Mechanism: The behavior of sodium sul- were otherwise without effect on the passage of salt fate was documented using saturated solutions and solutions. Only alkali ferrocyanides were found to im- oolites from Monks Park limestone (a stone well pact significantly on the interaction of these solutions known to be vulnerable to salt damage) as test sam- as they moved through the limestone. The addition of ples. Damage was evaluated based on the degree of 0.01-1.00% of K4Fe(CN)6 to sodium chloride in cracking of the oolite. Observations of sodium sulfate Monks Park limestone experiments increased the flow behavior using the environmental scanning electron rate of solutions through the stone, resulting in microscope (ESEM) and time-lapse video methods slightly yellow-colored efflorescence in place of subflo- show that damage during wet/dry cycling generally rescence, and yielded a massive formation of extended only occurs after larger mirabilite crystals (~10 mi- dendritic filaments without damaging the stone. This crons) undergo dehydration to thenardite, producing a protection by additive was extended to sodium sulfate high-surface area substrate of sub-micron crystals with solutions, but only at lower salt concentrations. Re- strong capillary suction. When wetted, these thenardite sults comparable to the effect of adding K4Fe(CN)6 to crystals generally dissolve, rather than hydrate, result- concentrated sodium chloride Monks Park limestone ing in extremely rapid crystallization, as observed in experiments were obtained with saturated sodium sul- the ESEM. Simple crystallization and dissolution of fate solutions without additives by conducting the mirabilite, without dehydration appears to cause little experiments in a draft-free, high humidity environ- damage after 10 cycles. ment--suggesting a potentially useful strategy for the Salt damage theory suggests that the limiting fac- conservation of fragile, salt-laden objects. These results tor in causing damage is the need to sustain a solution are explained by factors causing evaporation of solu- in situ with a high supersaturation ratio [3]. This is tion to occur either below or at the surface of the stone, generally difficult in many porous materials since their and by the effect of modifiers on the crystal habit of naturally rough surfaces tend to enhance crystallization the salts forming during evaporation in this region. once saturation is reached. Rapid cooling is one way to References: accomplish high supersaturation ratio in the sodium [1] Doehne E. (1994) In situ dynamics of sodium sul- sulfate system. However, our in situ observations sug- fate hydration and dehydration in stone pores: Observa- gest that the process of cyclic dehydration followed by tions at high magnification using the environmental contact with a saturated solution of sodium sulfate can scanning electron microscope, in Fassina V., ed., The result in substantial damage to porous materials. Miti- Conservation of Monuments in the Mediterranean gation methods such as limiting the rapid cooling or Basin, 143-150. [2] Rodriguez-Navarro C., Doehne E., rapid drying of porous, salt-laden stone through the and Sebastian E. (2000) Cement and Concrete Re- use of shelters may reduce the damage potential of search 30, 1527-1534. [3] Rodriguez-Navarro C. and sodium sulfate. Doehne E. (1999) Earth Surf. Processes Landforms Reducing Damage: Crystallization modifiers can 24, 191-209. [4] Rodriguez-Navarro C., Doehne E., significantly effect the capillary passage of dilute and and Sebastian E. (1999) Bulletin of the Geological concentrated solutions of sodium chloride and sodium Society of America 111, 1250-1255.