Posts Tagged ‘crust’

Under Construction, August 2011

1. BACKGROUND

Many Alaska Native cultures have used bone, tooth, ivory and antler extensively in their tools, fishing and hunting gear, ornaments, and other items. Some of these materials may look similar if they come from an archaeological setting.  Marine mammals provided materials common to Alaska but not widely seen in many other places, such as whale vertebrae and walrus tusk ivory. Mammoth and mastodon ivory is also sometimes seen made into artifacts. Beware, material called “whalebone” is sometimes actually referring to baleen, the filtering mouth parts of certain whales.  This material looks a bit like black or brown plastic and was widely used as corset stays, for example. It is made of the protein keratin and is not actually bone at all.

2. POSSIBLE CAUSES

The most common white stuff we have seen on Alaskan bone, tooth, ivory and antler is related to exhibition or repair, such as adhesives and putties. Burned bone, tooth, ivory or antler may be “calcined” or oxidized by heating which can cause a white powdery or crusty material. Sometimes, in the case of archaeological material for example, darker surfaces chip off or abrade away and reveal lighter white-looking areas below.  This is most commonly seen on antler from archaeological contexts. The walrus tusk container seen in the image below has “white stuff” in the incised lines of the carving. The records suggest the artifact came from an archaeological context on St Lawrence Island and was sold to the Museum by a resident many years ago. The white material may have been rubbed into the incised lines at that time to help highlight the design.

3. REFERENCES

Johnson, Jessica S. “Consolidation of Archaeological Bone: A Conservation Perspective.”  Jourbal of Field Archaeology.  Vol. 21, 1994.

Ellen Carrlee’s notes: Explores consolidants used by conservators in the mid 1990’s (many of which are still used) to consolidate bone, including acrylic resins Acryloid B-72, water based acrylic colloidal dispersion Acrysol WS-24, water based acrylic emulsion such as Rhoplex AC-33, poly(vinyl) acetate resin such as AYAA or AYAF, poly(vinyl) butyral resin Butvar B-98.  Also describes consolidants that have been used in the past but are not now recommended, such as “white glue” polyvinyl acetate emulsion (Elmer’s Glue, Carpenter’s Glue), wax, shellac, cellulose nitrate (marketed as Duco or Ambroid), gum dammar, gum Arabic, polyethylene glycol (PEG or Carbowax), agar jelly, ethylhydrohyethycellulose, poly(vinyl) acetate emulsion (Vinamul or Gelva), poly(vinyl) acetate resin (Vinylite or Gelva), epoxy, cyanoacrylate “crazy glue” (marketed today as Paleobond, for example, and known to be used in Alaska). 

Koob, Stephen P. (1984) “The Consolidation of Archaeological Bone.”  Adhesives and Consolidants. Preprints of the Contributions to the IIC Paris Congress, 2-8 september 1984 London. pp. 98-102.

4. EXAMPLES IN ALASKA

Under Construction August 2011

1. BACKGROUND

Glass structure is composed primarily of silicates and can remain stable and relatively unchanged for hundreds, if not thousands, of years if cared for properly.  Unfortunately, even with the proper care, this is not always the case as some glass is known to deteriorate rapidly over time.

 On a chemical level, most glass is stable.  However, some glass artifacts are known to undergo a complex disintegration.  This chemical breakdown of glass is commonly known as glass disease.  When discussed in reference to the most ubiquitous of archaeological and ethnographic artifacts, the glass bead, it is often termed ‘bead disease’.

The main component in glass is silicon dioxide, also known as silica.  Silica occurs naturally in three forms.  Its solid form is known as quartz, its amorphous or non-crystalline form is known as opal, and it is commonly found in its impure form as sand.  Glass can be made from pure silica, but it has a very high melting point of 4,172 ° F – which makes it rather difficult to work with directly.  For this reason, most glass mixtures have traditionally contained 70-74% silica and 16-22% of an alkali material, which serves to lower the glass melting point. Quite often, the alkali material used was either soda ash, a sodium carbonate which is obtained from burnt plant material, or potash, a potassium carbonate usually derived from wood ash. 

The sodium carbonate in soda ash produces a clearer glass than potash, so it was – and still is – more commonly used in glass manufacture.  However, when sodium carbonate is added to silica, the resulting glass is water soluble – meaning it will dissolve in water.  This is generally an undesirable characteristic for glass.  For that reason, lime (calcium oxide) is often added along with other minerals for better durability.  The addition of lime also helps the different components to mix together more easily.

2. POSSIBLE CAUSES

The most common white stuff we have seen on Alaskan glass artifacts is glass disease (crizzling, weeping) on beads. Abrasion, ghosting, mold, and previous treatments have also been seen on glass.

Glass Disease

While glass disease is seen on various kinds of glass objects, the most common Alaskan artifacts we have seen it on are glass beads. This damage might be historically interesting, as it might suggest a very old bead.

By the 16th century in western Europe, the production of glass (and beads in particular) became central to the economic and political endeavors of the time.  According to Lois Sherr Dubin, (2009, 29) “For Europeans, whose aim was to maintain maximum profits with a minimum commitment of manpower and resources, glass beads, exchanged for American furs or African ivory, gold, and slaves, yielded enormous margins – 1,000 percent was the return on investment, according to a report in 1632 – and thus became a central part of international trade patterns.”  

For a long time Venice was the center of glass bead production in Europe.  In 1292 A.D., glass production was moved from Venice to the nearby island of Murano in an effort to keep their production methods completely secret as well as to protect Venice from the risk of fires from the now constantly burning glass furnaces.  Production continued to increase throughout the centuries and bead making techniques were adjusted to meet the increasing demand and to decrease production costs as much as possible.  Chemical compounds that were added to glass mixtures to lower the glass melting point began to be added in even larger amounts.   This reduced the amount of fuel needed to run the furnaces and sped up the process overall.

As mentioned above, most glass consists of approximately three-quarters silica with sodium carbonate added to lower the melting point and calcium oxide to stabilize the mixture. If this combination is kept in balance, the glass is likely to remain stable. However, when there is an imbalance in the proportion of these components, problems can arise.  If there is an excess of alkali and too little lime (as was happening in Murano), the surface of the glass may begin to react with moisture in the air and start to break down.  This is the major cause of glass disease.

Glass disease is therefore inherent in the chemical makeup of certain glasses.  This is both good and bad news.  The good news is that it can’t spread to other glass in your collection– it’s not contagious. The bad news is that if the chemical composition of the bead lends itself to glass disease, there is nothing that can be done to stop it from breaking down.  The corrosive nature of glass disease causes a snowball effect of sorts on objects that succumb to it.  Once the process begins, there is no known treatment that can reverse the effects or stop it from proceeding.  The degradation can be slowed down considerably with cleaning and careful monitoring of the storage environment, but nothing will “cure” it, so to speak.

At the present time, it is not fully understood how this decomposition of glass proceeds on a molecular level.  However, we do know that in all glass the sodium and potassium carbonates are hygroscopic.  This means that they readily take up and retain moisture from the air.  Once these salts become hydrated, they can leach out of the glass and form crusty deposits on the surface.

As the sodium and potassium ions are removed from the chemical structure they are replaced by hydrogen ions, which diffuse throughout the glass.  This creates a hydrated silica network which is inherently weaker.  As the deterioration of the glass progresses, the surface of the glass becomes increasingly alkaline.  An alkaline substance is one the measures above 7 on the pH scale.

A bead with glass disease will show many symptoms, some of which can be seen easily and others that may require the use of a magnifying lens or microscope.  There are five signs of glass disease which are commonly cited by conservators.  These are: broken beads, sweating or weeping beads, white crusty deposits, crizzling, and damage to the backing material.  Additionally, as glass begins to deteriorate, it will quite often have a dull, foggy appearance.  This results from deposits left on the surface of the glass, as well as crizzling altering the reflectivity of the glass.

In certain environments, droplets of moisture may appear on the surface of a glass bead.  This is known as sweating or weeping.  This occurs when atmospheric moisture combines with the alkali material used in the manufacture of the glass and causes the hygroscopic alkali salts leech out.  These salts migrate to the surface forming a soapy, sticky alkaline solution.  This soapy residue forms abrasive and caustic by-products, which draws dust and dirt to the surface.  This in turn attracts more moisture to the glass and facilitates the progression of the glass bead deterioration.  Glass beads may have a white, fuzzy look to them as salts from these residues crystallize, as well as from the dust and dirt attracted to the soapy alkaline residue. 

Unstable glass and high humidity can also result in the formation of crusty deposits on the surface of beads.  As discussed earlier, alkaline products that migrate out of the beads turn into alkaline salts, which are left on the surface of the bead or adjacent material.  This produces a hard alkaline coating which can give glass a white, dusty appearance.

As the deterioration of unstable glass progress, small fissures in the surface of the glass start to become visible.  This is known as crizzling and it is characterized by a fine all-over cracking or fracturing of glass.  This step in the degradation of the glass can also lead to flaking and pitting on the surface of the bead.  Crizzling of the glass surface can cause transparent beads to look opaque and also contributes to the appearance of a whitish haze.

It is not known for sure whether or not glass disease occurs in some colors and/or sizes of beads more than others, but our experience through this survey of Alaskan beads is that it might.

Abrasion/scratches

The most common problem associated with caring for historic glass, including glass beads, is their fragility.  Like all glass objects, glass beads can crack, break, or become easily scratched if they, or the objects to which they are attached, are not carefully handled. Abrasion and scratches interfere with the way light passes through the glass and can cause a whitish, opaque haze. Think of glass fishing floats, for example, which can appear hazy if they have been abraded by sand.

Ghosting or transferred images

The reason that transferred images are sometimes referred to as “ghosting” is because a replica of the image can usually be seen in the glass. These images and other blooms on paintings are sometimes attributed to mold due to the microenvironment that exists between a painting and it’s glazing (Williams 1988, 66). It seems rather unlikely, however, to have a mold that grew in such a specific pattern as to duplicate an image. When the white accretions making up transferred images were analyzed in the 1980s, they were discovered to be made primarily from ketones and sodium soaps (Williams 1988, 69). The overall hypothesis for this is that ketones volatilize from the paint, condense on the glass and then oxidize into carboxylic acids. These acids then react with sodium in the glass to form the sodium soaps that make up the images (Williams 1988, 70). This is a similar reaction to that found happening in glass beads – particularly those in contact with lipid-containing ethnographic materials. It’s worth noting that in many cases where an artwork came into contact with glass, no transfer image was formed. Rather, a halo of clear glass remained around the point of contact with the transfer image forming again beyond this halo (Williams 1988, 68).  Williams describes the primary melting point of the transfer image material analyzed as being consistently at 69°-70° C (1988, 67). Since the transfer images are composed of organic compounds, they have poor solubility in water. To test this, take a small sample of the white stuff and place it on a glass slide or similar. Add a couple drops of water. If the compound does not appear to dissolve, this would rule out a soluble salt and may indicate the presence of insoluble organic compound.

mold

Mold is typically described as having a fuzzy, velvety, or sometimes slimy appearance. Mold needs organic material to feed on, so glass doesn’t commonly provide the necessary ingredients. Glass used to frame a photo at the Alaska State Museum once had branch-like mold on the inside of the glass, suggesting the gelatin of the photo might have provided enough nutrients for mold. Glass beads strung on sinew or cotton thread might also provide adequate nutrition and you might see mold growing out of and around the beads – especially if the threads/sinew have absorbed moisture and created high RH microenvironments inside of the beads. Additionally, dust and grime that accumulate on glass can provide the necessary materials to allow for mold growth. Mold was once seen on the glass touching the surface of a framed photograph at the Alaska State Museum. When viewed under a microscope, the vegetative part of mold (known as mycelium http://en.wikipedia.org/wiki/Mycelium) can be seen as thin, thread-like branching hyphae and is very distinctive from the crystalline structure of salts. Mold growth generally begins to occur on organic materials when the environment is at 70% relative humidity or higher. The Canadian Conservation Institute (CCI) gives the following useful chart for mold growth on their “10 Agents of Deterioration” website http://www.cci-icc.gc.ca/crc/articles/mcpm/chap10-eng.aspx:

Previous treatment

Different types of treatments may have been used in the past on glass beaded objects that could cause a white or hazy appearance. Adhesives may have been used to reattach beads, and these often can look cloudy and opaque as they age. Pesticide residues can create a whitish haze over the surface of materials, including beads. Pesticides were commonly applied during the early 20th century to collections containing natural history specimens and ethnographic artifacts made of organic materials such as leather, fur, and feathers.

3. REFERENCES

Mold hyphae, image by Bob Blaylock

http://en.wikipedia.org/wiki/File:20100815_1818_Mold.jpg

Carroll, Scott and Kelly McHugh.  (1999) “Material Characterization of Glass Disease on Beaded Ethnographic Artifacts from the Collection of the National Museum of the American Indian.”  Ethnographic beadwork: Aspects of Manufacture, Use and Conservation. Edited by Margot M. Wright. Conservation Centre, National Museums and Galleries on Merseyside on 22 July 1999

Ellen Carrlee’s notes: Most impacted beads fell into one of two categories: waxy/crusty beads that tested positive for triglyceride oils and powdery/crystalline beads that did not.  Sometimes kaolin clays used to whiten leather show up as white powder on beads but are not harmful.  Spot testing was also able to ID chlorides on beads throught to have contact with salt water.

Dubin, Lois Sherr (2009) The History of Beads from 100,000 to the Present.  Harry N Abrams, Inc.  New York.

Fenn, Julia.  (1987) “Deterioration of Glass Trade Beads in Contact with Skin and Leather or Glass Beads in Soapy Bubble.”  ICOM Committee for Conservation 8th Triennial Meeting, Working Group 3 Ethnographic Materials. Sydney Australia. Pp 195-197.

Jenkins, Michael R. “Glass Trade Beads in Alaska.” Alaska Journal  2.3 (1972), 31-119.

Crista Pack’s notes: Date for beads entering Alaska is unknown; however trade may have brought them into the region long before the first contact with non-indigenous explorers. Gives an insightful and thorough history of bead trading and the value of beads along the NW coast. Provides many images of different types of bead and objects beads were used in the manufacture of.

Lougheed, S. (1988) “Deteriorating Glass Beads on Ethnographic Objects: Symptoms and Conservation.”  Symposium 86: The Care and Preservation of Ethnographic Materials Editors R. Barclay et al Ottawa, Canadian Conservation Institute. Pp. 109-113.

Ellen Carrlee’s notes:  Controlling RH is the best solution for beads that exhibit early symptoms of glass dieases such as broken beads, sweating beads, crusts on bead or thread, crizzling, bleached spot below the bead on a textile, darkening of leather in contact with the bead. RH should be between 35% and 42% ought to control it.  Below 30% crizzling may occur, and above 42% alkaline carbonates that leach to the surface become hygroscopic and accelerate the process.

Ordonez, Eugenia and John Twilley, John.  (1998) “Clarifying the Haze: Efflorescence on Works of Art” WAAC Newsletter 20 (1) 1998 pp 12-17.

Pearson, C. (1987) “Deterioration of Ceramic, Glass and Stone” Conservation of Marine Archaeological Objects.  Butterworths.  London.

Sirois, P. Jane. (1999).  “The Deterioration of Glass Trade Beads from Canadian Ethnographic and Textile Collections.”  The Conservation of Glass and Ceramics: Research, Practice and Training.  Norman H. Tennant editor.  James & James.  London. Pp. 84-95

Williams, R. Scott. (1989) “Blooms, Blushes, Transferred Images and Mouldy Surfaces: What Are These Distracting Accretions on Art Works?”  In Proceedings of the 14th Annual IIC-CG Conference 1988.  Edited by Johanna G. Wellheiser. Ottawa. Pp 65-84

4. EXAMPLES IN ALASKA

Under Construction August 2011

1. BACKGROUND

There is a wide variety of metal artifacts in Alaskan museum collections.  Tlingit copper daggers and carved silver jewelry, Siberian Yupik lead pipe bowls, Russian axe heads, aircraft engines and maritime artifacts are but a few examples.

2. POSSIBLE CAUSES

The most common white stuff we have seen on Alaskan metal artifacts is corrosion. The most common white corrosion products are found on zinc (so-called white rust), lead, pewter, tin, and aluminum. Also seen on metals are polishing residues, which can cause corrosion stress cracking in brass if those polishing compounds contain ammonia. Those polishing residues often have a greenish white look and are in the crevices or carving details of the metal. Metals in association with leather sometimes corrode due to the oils present in the leather. Lead corrodes easily in the presence of certain pollutants, particularly volatile organic acids that may come from other collections artifacts or poor collections storage materials. In fact, a fresh lead coupon in a jar with suspected off-gassing materials is part of the so-called “Oddy Test” for poor storage materials. If white stuff appears on the lead, the material in question may be releasing pollutants. Siberian Yupik pipes are typically made with lead and many in the Alaska State Museum collection are corroding. The often have small leather or cloth pouches attached, and those sometimes have white crystalline formations on the outside.  We have tested one for lead, and come up negative.  However, the tobacco mixture sometimes contained birch tree fungus ash, assort of a potash.  Potash is chemically rich in potassium, and we did get a nice peak for potassium on our XRF analysis so possibly those white crystals on the pouch are related to the tobacco that was kept inside.  Some Tlingit rattles have been found to be filled with small pellets of lead shot to make the rattling sound. Aleut kayak models in our collection have kayaker hands made of lead, and those have corroded aggressively, including after treatment with a corrosion inhibitor called Incralac and coating with Acryloid B-72.  Remember that lead corrosion is poisonous, so wear gloves when handling.  We have also seen the drawer runners on some of our collections cabinets corroding, but have had trouble determining what the white corrosion product is and why those drawer runners are corroding.  We don’t want that white powder getting on our artifacts.  Similar white powder has been seen on aircraft engine parts in the collection, but those parts are likely made of aluminum. An interesting white metal issue we have not observed but may be possible in some collections is tin pest.  Tin usually appears as its stable beta form allotrope, but at low temperature pure tin can transform into another less stable form and become white and crumbly.  Tin must be fairly pure in order for this to happen, however, so it is rare. According to information added to Wikipedia in the past several years, the temperature needed is usually below 56F and may need to be sustained for over a year. Tin solder on cans, uniform buttons, and medieval organ pipes are cited as examples.  Different metals in contact with each other may experience galvanic corrosion.  In this case, one metal acts as an anode and the other as a cathode in the presence of an electrolyte. Zinc and aluminum for example, are often used intentionally as a sacrificial anode to corrode preferentially to more noble metals they are protecting. This situation can happen accidentally as well. 

3. REFERENCES

Selwyn, Lyndsie. (2004) Metals and Corrosion: A Handbook for the Conservation Professional.  Canadian Conservation Institute.  Ottawa.

Tennant, Norman H, Brian G. Cooksey, David Littlejohn, Barbara J. Ottaway, Steven E. Tarling, and Martin Vickers.  (1993) “Unusual Corrosion and Efflorescence Products on Bronze and Iron Antiquities Stored in Wooden Cabinets.” In, Conservation Science in the UK: Preprints of the Meeting held in Glasgow, May 1993.  Editor: Norman H. Tennant.  James & James Science.  London. Pp. 60-66.

Ellen Carrlee’s notes: there are many conservation articles on obscure and strange corrosion products on metals.  The small amount of sample usually available and the range of possible corrosion products makes identification of the corrosion product challenging and pinpointing the cause even more difficult.

4. EXAMPLES IN ALASKA

Under Construction, August 2011

1. BACKGROUND

Baskets are very common in Alaska, and are often used where ceramics might have been common in other cultures. Typically, baskets are made of plant materials such as spruce root, cedar bark, birch bark, or grasses. Archaeological basketry over 5,000 years old has also been found in waterlogged sites in Southeast Alaska, and several hundred years old on Kodiak Island.

2. POSSIBLE CAUSES

The most common white stuff we have seen on Alaskan baskets are dust, mold, adhesives, paint spatters, insect debris (such as cocoons) and PEG (polyethylene glycol.) Look with a magnifying glass to see how the white stuff is deposited. Powdery-looking spotty deposits may be mold. Dust would likely settle on certain areas that are horizontal, such as the lid if it has one or inside the base. The underside of the base may have accretions from adhesives, labels, or unclean shelves. Baskets were sometimes adhered onto an exhibit shelf in the old days to prevent them from moving with vibration of footsteps. Adhesives and repairs of various kinds have been used on baskets, so white stuff in association with a tear or loss is likely an adhesive. Waterlogged archaeological basketry was most commonly treated with a white glue in the 1960’s and 70’s, but since then polyethylene glycol treatments have been more typical.  Too much high molecular weight PEG (PEG 3350 or PEG 4000 for example) will result in white deposits on the surface.  These are soluble in warm water, and you can test this with a barely-damp cotton swab on the surface.  There was a period of time when “feeding” baskets with oil was a popular maintenance technique. This sometimes appears as white haze on baskets, and may also make them brittle. Haze could also be a pesticide residue. Always be careful to wear gloves…not only are you protecting the baskets from substances on your hands, you are protecting yourself from whatever may be on the basket.

3. REFERENCES

Hartley, Emily. (1978) The Care and Feeding of Baskets. Self-published.

Ellen Carrlee’s notes: Coating mentioned is paraffin oil in mineral spirits, 16% solution, p 29.  The author mentions that the techniques are derived from procedures developed by Bethune Gibson and Carolyn Rose of the Anthropology Conservation Lab of the Smithsonian’s Natural History Museum, and used there around 1974-75.

4. EXAMPLES IN ALASKA (click to enlarge images and see more info)

Conservators at the Alaska State Museum are regularly asked this question:

“What’s that white stuff?”

Disfiguring white stuff on artifacts and artworks can be the result of a wide range of causes. These are referred to by a variety of terms including: efflorescence, bloom, fatty bloom, spue, blushing, chalking, blanching, haze, dust, grime, salts, mold, transferred images, crizzling, sweating and patina. While these terms each refer to specific types of damages and disfigurements, they are often used interchangeably. This may be due to the similar appearance many of them have at first glance. White fuzzy material on an artifact is often assumed to be some type of mold. Sometimes this is the case, but a number of other things can have the same appearance. Staff at the Sheldon Jackson Museum were concerned that Tlingit hide armor was breaking out with mold. Small white fuzzy patches were disfiguring the surface, and the problem seemed to be getting worse. However, upon examination, the white substance was waxy and records indicated a leather dressing treatment was undertaken in the late 1960’s. At the time, leather dressings were a common part of museum practice and believed to benefit leather by aiding flexibility. Over time, the fats can alter and come out of the leather as a bloom or spue, disfiguring the surface. Leather dressings are no longer part of museum conservation practice.

A wooden dish in the Alaska State Museum collection has a fine sugary-looking white deposit inside. Examination and history of the dish reveal the crystals are related to the bowl’s history as a vessel for holding grease or oil. The presence of those crystals is part of the history of the artifact and tells a story.

GETTING STARTED

“What’s that white stuff” is a great question. Does it belong there? Is it hazardous to humans? Is it destroying the artifact? We suggest a systematic approach to the problem.

  1. Identify the material affected. What is your artifact made of? Our INDEX lists artifact materials vulnerable to “white stuff.”
  2. Examine the context and history of the artifact. What was it used for? How was it maintained? Was it dug out of the ground? Has the environment been stable? Has it been on exhibit? Also ask folks who have worked at your museum a long time. These clues may help narrow down possibilities. Write them down.
  3. Characterize the appearance of the white stuff. Is it powdery? Sticky? Flaky? Is it a haze or a crust? Does it appear in a pattern? Use our list of descriptive words below to help you. Write it down. Take a photo for the files.
  4. Consider the typical POSSIBILITIES. For each kind of material, there are certain kinds of white stuff that we have see more than others in Alaskan collections. We try to list these in the INDEX by material.
  5. Test the hypothesis. Make a guess at what you think it is, and if you can remove a little bit, test your theory.

HOW DO I DESCRIBE WHAT I SEE?

Here’s a list of vocabulary words that can help you characterize what you are looking at.  These words, plus a photo, can help others work on solving your mystery. Check GLOSSARY if needed.

airy

branched

chalky

cloudy

crumbly

crusty

crystalline

dotted

dry

fibrous

filmy

foamy

fuzzy

glossy

grainy

greasy

hard

hazy

matte

opaque

powdery

rough

spidery

spotty

sticky

soft

sugary

translucent

HOW CAN I ANALYZE MY WHITE STUFF?

EASY

  1. Magnifying glass and strong light. Look carefully at the surface and try to characterize the appearance. Is it an optical effect from delamination or abrasion? Or is it accumulated on the surface, indicating an accretion or efflorescence?
  2. Does it roll easily on a tipped surface, like tiny dry balls?  You may have insect frass.
  3. Look for patterns on the surface or an explanation for why the white stuff is in some areas and not others. Consider different materials, which side is “up” and if the pattern may be associated with something applied as a liquid.
  4. If you can sample it with a small pointy tool, note how difficult it is to remove. Powdery? Crumbly? Sticky? Smeary?  This is an important clue. Don’t remove it all during sampling, you may want to try again with another idea.
  5. If you have removed a bit of it, try rubbing some of it on a clean glass surface. Does it smear? Or does it stay crumbly/powdery? A smearing substance can indicate the presences of fats or waxes and may indicate the sample is some type of fatty bloom.
  6. Try adding a drop or two of water, enough to cover the sample. Does it dissolve? If so, it may be a water-soluble salt.
  7. With another sample, try adding a drop or two of mineral spirits.  Does it dissolve now?  Perhaps it is something waxy.
  8. If the sample melts with gentle heating, it may be a wax or bloom. Melting can be done over a hotplate, or even with a bit of foil over a candle.

TRICKY:

  1. If you have a binocular microscope, you can see more detail. For example, branching structures of mold may be visible. Crystals may be evident. Bug parts are more obvious under magnification.
  2. If you have a polarized light microscope, you can see even more detail and observe optical properties. Fruiting bodies confirming mold may be present.

HARD:

  1. Various kind of instrumental analysis may be available at larger museums or universities.  For example, an XRF can identify certain heavier elements that point to specific possibilities like metal corrosion, pesticides etc.
  2. If you are equipped to use acids safely, add a couple of drops of HCl. If your sample bubbles vigorously, it may be a carbonate. Maritime accretions and insoluble archaeological salts often bubble with this test.
  3. While some resources may suggest tasting the salt to confirm that is what it is…this is not a good idea and tasting should never be done to identify an unknown substance. The salts forms in Byne’s disease, for example, are not components of common table salt (sodium chloride) and should not be consumed.
  4. Spot testing can help characterize “white stuff” but you must be equipped with a chemistry lab and familiar with laboratory techniques to perform many of these tests. The main museum reference for these tests is:

Odegaard, Nancy, Scott Carroll, and Werner S. Zimmt. (2005) Material Characeterization Tests for Objects of Art and Archaeology.  Second Edition. Archetype Publications Limited. London.