conserveogram - soluble salts and deterioration of archaeological materials

8/2/2019 ConserveOGram - Soluble Salts and Deterioration of Archaeological Materials

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Soluble Salts And Deterioration Of Archeological MaterialsPorous archeological artifacts such as ceramics,stone, bone, and ivory often contain soluble salts.Ground water and seawater can carry these saltsinto the pores of the artifact during burial leavingthem behind when the water evaporates. Afterexcavation, these salts can crystallize at or justbelow the surface of the artifact causing damage.A variety of descriptive terms are used for thisdamage including spalling , flaking, powdering,and sugaring. Identification may be difficult ifthis damage is done to the painted, polished orslipped surface of an object. The force ofgrowing crystals can break apart the surface ofbone, stone, ceramics and other porous materialsso that detail is lost. In bad cases it can removethe entire surface of an artifact. In the worstcases, it can destroy an artifact. This Conserve 0Gram will help you identify salts and discusstreatments and storage alternatives.

SURFACE LAYER

POROUS SUBSTRATE

Figure 1. Sket ch of a porous object w ith undam agedsugace layer.

Soluble Salts and Insoluble SaltsConservators divide the salts that are deposited inand on an artifact during burial into two groups:insoluble salts and soluble salts. Soluble saltswill dissolve in moisture in the air. This propertyis known as deliquescence. The salts can movethrough the porous structure of an artifact asmoisture is drawn out through evaporation. Asthe salts reach the surface of the artifact they maycrystallize as white, often furry growths on thesurface. If the surface is less porous than theunderlying structure they can crystallize justbelow the surface. These crystals exert immensepressure and may cause the surface layer to spa11off.

DAMAGED SURFACE LAYER SALT CRYSTALS.I

POROUS SUBSTRATE

Figure 2. Damage caused w hen salt draw n out ofpores crystallizes and pushes off the surface lay er.


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Insoluble salts are not truly insoluble but willtake days or weeks to dissolve in water. Theyare not deliquescent and so will not cause furtherdamage after excavation. Insoluble salts can,however, be quite disfiguring, and may requireremoval for identification or reconstruction of anartifact. Common salts are listed in Table 1.

be visible as tall fibers extending up from thesurface.

Soluble Salts Insoluble SaltsChlorides CarbonatesNitrates SulphidesSulfates Phosphates

Table 1. Common salts affecting porous materialsfrom archeological sites

Soluble salts can also be deposited into artifactsthrough past conservation treatments, includingacid cleaning. In this process, objects aredipped in a dilute acid solution to removeinsoluble salt deposits from the surface ofceramics. The acid changes them into a form thatis water-soluble. The soluble salts can thenpenetrate into the porous body of the artifact andlater recrystallize on or under the surface asdescribed above. Acid residues may also reactwith the artifact causing crystal growth.

In contrast, insoluble salts usually have a lumpy,uneven appearance that contains contaminantsfrom the surrounding soil. They may be laiddown in long lines following root growth. Theymay obscure the whole surface of the artifact.They are usually hard. Since they usually havebeen deposited underground they oftenincorporate sand grains or small pebbles and mayhave a muddy appearance. These salts will notdissolve in atmospheric moisture, and will notcause any further deterioration over time.Insoluble salts are also known informally ascaliche and marl. Salts can also form on artifacts due to reactionwith storage materials. Acids released fromwoods and paints, among other things, can reactwith components of the artifacts causing crystalgrowth.

Zdentifling Salts

In order to identify the salts conservators useanalytical methods such as spot tests or x-raydiffraction. Identification of salts can helppinpoint whether a problem comes from previoustreatment or improper storage.

Soluble salts are visible as a white growth on thesurface of an artifact. In newly excavatedmaterial, they often form first along cracks orabraded areas of a surface. Often they can looklike a white bloom or haze on the surface. As thecrystals continue to grow and form they willextend further from the surface and appear as awhite powder or even look somewhat like tablesalt. They may have a soft, fuzzy feel if touched.

TraditionalTreatment or Soluble Salt sThere are problems with the traditional techniquefor removing soluble salts. However, no goodalternatives have been developed so conservatorsand archeologists still commonly use thistechnique. Many artifacts that come intocollections or that are housed in collections nowmay have gone through the soaking treatmentdescribed below.

If the soluble salts have grown in storage, they The method is described here to identify theare often first noticed when powder and small problems with this technique. Any treatment willfragments are seen on the shelf around the cause changes to archeological objects that mayartifact. They may show up more clearly if limit their usefulness for analysis in the future.viewed with raking light (light shining at a low Conservators typically weigh the drawbacks ofangle from the side). If the crystal growth has any treatment against the likelihood of physicaltaken place undisturbed for a long time, they can deterioration of the object if no treatment is done.

2 Soluble Salts ancl Deterioration of Archeological Materials


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The best course of action may be to not desalinateat all.To remove soluble salts from artifacts thetraditional technique has been to soak the artifactin water. This technique is not appropriate forartifacts with fragile surfaces. A typicaltreatment is carried out as follows:1. The artifact is immersed in a known volume

of deionized or distilled water and left tosoak.

l probably dependent on many factors,including the exact types of salts present, theporosity of the object and the environmentalconditions in which the object is to be kept.

l It may take gallons of deionized or distilledwater to desalinate an artifact if this treatmentis carried out in the field. Clean water can behard to acquire in a field lab.

Museum Storage for Objects ContainingSoluble Salts

2 . After a set amount of time, typically 24hours, the salt level is measured using thesilver nitrate test (see Conserve 0 Gram 613)or a conductivity meter to identify the levelsof salt that have been removed from theceramic.

3. The old water is discarded and clean water isreplaced to the same level as before.

4. These steps are continued until the amount ofmeasured salt has leveled off or reached alow level.

There are several problems with this technique.

In a museum context, inorganic materials likeceramics, bone, or stone are often consideredhardier than wooden or textile objects. However,because of the contaminants introduced duringburial, porous, inorganic, archeological objectsalso require environmentally controlled storage.Soluble salts from burial or previous treatmentcan be controlled through storage in a stablerelative humidity. Fluctuations in RH can movethe deliquescent salts in and out of solution,which draws the salts up through the pores of theartifact. At a high RH they will be dissolved.When the RH drops they will crystallize. Tolimit salt movement, keep the RH stable.

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Soluble Sa lts and Deterioration of Archeological Mat erials 3

Desalination can take many days or evenweeks. Soaking for days can removecomponents of the original ceramic or stone,use residues, or organic and inorganicconstituents of bone. This can affect theusefulness of these materials for analyses inthe future and can structurally weaken theartifact.

If active deterioration is found on objects instorage a conservator should be contacted.Actively deteriorating objects will have fragilesurfaces. Salts may be seen as a white bloomor as crystals. Small fragments of material maylitter the surface around the object.

There is no generally agreed upon standardfor what level of salt removal is enough. Noone knows how much salt in an artifact isnecessary to cause damage and the level mayvary from object to object. Recent studiessuggest that what have typically beenconsidered to be safe levels may bedrastically lower than necessary. This level is

To stop salt deterioration, once it has begun,consult with a conservator who will help youdevelop a preservation strategy that takes intoaccount the material and the storage environment.There are a variety of preventive or interventivetechniques that may be appropriate. Theseinclude improving the storage environment,creating stable RH microenvironments, ortreatment, depending on the particular situation.


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ReferencesCronyn, J. M. The Elements of ArchaeologicalConservation. London: Routledge, 1990.Fitzhugh, Elizabeth West and Rutherford J.Gettens. Calclacite and other efflorescent saltson objects stored in wooden museum cases. InScience and Archaeology, edited by R.H. Brill.Cambridge, Mass: M.I.T. Press.Hodges, Henry W .M. The conservationtreatment of ceramics in the field. In In SituArchaeological Conservat ion, Proceedings ofMeetings, April 6-13, 1986, Mexico, edited byHenry W .M. Hodges. Mexico: InstitutoNational de Antropologia e Historia de Mexicoand Century City, CA: J. Paul Getty Trust, 1987.Sease, Catherine. A Conservation M anual for theField Archaeologist. Los Angeles: Institute ofArchaeology, UCLA, 1992.Torraca, G. Porous Building Materials -Materials Science for Architectural Conservation,2nded. Rome: ICCROM, 1975.Jessica S. JohnsonConservatorMuseum Management ProgramNational Park ServiceWashington, DC 20240

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