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Lab 1: Artifact Documentation

  • Background information on
    • Clay and clay bodies
    • Glazes
    • Why they are important
  • Documentation of pottery and ceramics shards
    • Written description
    • Mohs’ Hardness Scale
  • Measurement
    • Metric System versus English System
    • Online conversion chart
    • Online conversion calculator
  • Photography
  • Lab 1: Artifact Documention Worksheet

Background

Clay and Clay Bodies

Clays are formed through the weathering of certain minerals.  They are made up of tetrahedral or octahedral sheets and have either no charge or a net negative charge that is balanced by cations such as Na+ or K+.  Because they are products of weathering, clays are usually found in sedimentary rocks such as shale or mudstone; and because of their small size, they are easily carried great distances by rivers or even by ocean currents.

The types of clays found in a certain location are dependent on the original rock types found in that area or upstream from that location.  In Louisiana, most clays have been deposited by the Mississippi River and its tributaries, either through flooding of those rivers or by changes in the location of the main channel of the river.

Clays have been used for a variety of purposes, some utilitarian, some artistic, and others purely industrial in nature.  For the majority of these purposes the plasticity, its ability to be shaped or formed, of a particular clay determines its usefulness.  Pure clays tend to shrink, crack, or warp if dried too quickly.  They also tend to fracture or even explode if heated too quickly during the firing process.  Adding a filler helps to minimize these problems.  Potters usually call this material “grog” while anthropologists call it “temper.”  Temper can be sand or just about any finely-ground material including shells, bone, rock, vegetable matter, or even ground pieces of previously fired clay.

When potters talk about the different types of clay used to make objects, they refer to these as different clay “bodies.”  Some clay bodies have different colors or handling characteristics.  They also “mature” at different temperatures. For example, porcelain and stoneware are fired to approximately 2300°F; earthenware is fired to a temperature of approximately 2000°F. Pit-firing, an early method of firing pottery, typically reaches a temperature of 1400°F to 1600°F.

During the lab exercise, you will examine pottery shards that served as either utilitarian or decorative purposes, and sometimes both.  Some are more recent, less than 200 years old, while others pre-date European settlement of the Louisiana area.

Glazes

Glazes serve both utilitarian and decorative purposes.  Some glazes are used to seal a clay body so that a vessel is water tight.  Other glazes are used to enhance the appearance of the form.

Glazes are often made up of the same kind of materials that make up clays, but include a flux to help the glaze melt in the firing process.  In lower-temperature firings, lead (Pb) was often used as the flux, while tin (Sn) was used to make the glaze white. The composition of the glaze also determines whether the glaze will appear shiny or matte once fired.  This surface texture can be adjusted by changing the proportions of chemicals that make up the glaze.

Decorative motifs on pottery are usually applied by brushing or painting on a “slip” that is made up of a metal oxide or a metal carbonate.  These slips can be applied over a glaze, in which case the slip will fuse with the glaze as the glaze melts, or they can be applied directly to the clay body and covered with a clear glaze.  One of the most common and reliable metal slip used in pottery is cobalt carbonate (CoCO3) which starts off as a pink colored slip and turns blue during firing.

Note that not all pottery includes a glaze.  Sometimes the form itself is decorative, or the decorative element(s) come from the construction method, surface treatment, or irregularities in the way the flames move across the pot during firing.

Why Clays and Glazes Are Important

The reason clays and glazes are important is because they can be used to help date objects. Additionally, clays and glazes can also give clues as to the origin of these objects.

Pottery and Ceramics Shards

The first lab exercise consists of three documentation parameters; written description, measurements, and photography. Before the advent of photography, archaeologists relied on well-written and detailed descriptions of artifacts. When describing a ceramic or pottery object, one must use the following conventions:

Written Description:

  • Color – Standardization is the key for the proper documentation of color. There is a method called Muncell™. This color standard comes in the form of a book and is very expensive. Archaeologists use this to grade dirt, pottery, ceramics, and other material culture. For the purpose of this exercise, we will simply describe the color of the artifact in a consistent manner. If one is consistent throughout the testing procedure, the results will be accurate.
    • Interior Color – Pottery and ceramics usually have an interior layer where the firing process did not penetrate.
    • Exterior Color – Pottery and ceramics can have a cloudy appearance which can be classified as ‘cloudy’ or having ‘mixed colors’.
  • Hardness – The terms used for this convention are as follows:
    • Soft – can be scratched with a fingernail.
    • Hard – cannot be scratched with a fingernail.
    • Very Hard – cannot be scratched with a knife.

Geologists use the Mohs’ hardness scale to identify minerals (figure 1). For our purposes today, we will only use the more subjective method.

Although these conventions are crude, they can give the researcher valuable clues as to the composition of the materials in question. One must realize that these tests should only be attempted on unprovenienced—samples that are not part of the archaeological record such as bits and pieces that have little or no cultural value—artifacts.

Scale Mohs Mineral Substitute
1 Talc
2 Gypsum
2.5 —– Fingernail
3 Calcite Copper wire
4 Fluorite
4.5 —– Window glass
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Sapphire
10 Diamond

Figure 1, Mohs’ Hardness Scale

  • Feel – Basic terms are:
    • Harsh – feels abrasive to the finger.
    • Rough – irregularities can be felt.
    • Smooth – no irregularities can be felt.
    • Soapy – slippery.
    • Powdery – a residue can be felt.
  • Texture – The following terms refer to freshly broken sections:
    • Subconchoidal – breaks like glass or flint.
    • Smooth – flat or slightly curved, no visible irregularities.
    • Fine – small closely spaced irregularities.
    • Irregular – larger, more widely spaced irregularities.
    • Hackly – large and generally angular irregularities.
    • Laminated – ‘stepped’ effect.

These descriptions usually can be done with the naked eye, but finer delineations may require a magnifying glass or stereo microscope.

  • Inclusions
    • A magnet can be used to identify ferrous particles.
    • A dilute acid can be used to identify limestone or shell particles.
    • The frequency of the inclusions is indicated by:
      • Abundant
      • Moderate
      • Sparse
    • Inclusions can include ‘temper’—material such as straw, shell, bone, etc.—which is used to bind the substrate or prevent breakage in the firing process.
    • Sizes of inclusions are standardized:
      • Very fine -      up to 0.1mm
      • Fine -              0.1 to 0.25mm
      • Medium -         0.25 to 0.5mm
      • Coarse -          0.5 to 1.00mm
      • Very coarse -  larger than 1.0mm
  • Rounding – The terms used are:
    • Angular – convex shape, sharp corners.
    • Sub-angular – convex shape, rounded-off corners.
    • Rounded – convex shape, no corners.
    • Irregular – convex/concave shape.
    • Flat – two-dimensional shape.
  • Surface Treatment – The terms used are:
    • Wiped – treated with a hard tool
    • Smoothed – a wet cloth use to smooth surface
    • Burnished – the surface has a sheen
    • Incised – grooved decorations
  • Glaze – Some glazes include:
    • Tin-glazed
    • Lead-glazed
    • Copper-stained glazed
  • Miscellaneous – some object represent animal or human figures. These are called:
    • Effigies

Measurement

When it comes to pottery and ceramics, measuring shards is not as important as a well written description. This is true because the shards are only parts of a whole where the ‘whole’ is the cultural object itself. One must record the following parameters:

  • Linear measurements (Metric System):
    • Length (average)
    • Width (average)
    • Thickness (average)
  • Volumetric measurements (Metric System)
    • Weight
    • Density

Why should we use the Metric System instead of the English System?

Conversion Chart.

Use a Conversion Calculator.

Photography

Photography is a valuable tool for documenting and evaluating pottery and ceramics. A photograph of an artifact can be taken in situ—in the place where it was found—or in the laboratory. Individual pottery shards can be recorded by photography as a means of illustrating lectures and workshops. Close-up photography is very useful for recording surface treatments and morphological characteristics. With the use of a scale and color chart, a photograph can be used for determining an object’s size and color. In this laboratory exercise we will use a photography stand with adjustable lighting, a centimeter scale, and a color chart.

IMPORTANT: Students are reminded that the bulbs on the photography stand are very hot and bright. Please do not touch or look directly at the light bulbs.

The following are a few pointers that will aid the student in the proper use of digital camera for documenting artifacts.

  • The artifact must be properly marked. There is usually an accession number somewhere on the artifact. Look very carefully and record that number in your workbook.
  • Each group of students will be issued a group number also. Please lay this number along with the centimeter scale, and color chart next to the artifact when photographing your artifact in the NCPTT lab.
  • The NCPTT digital camera is preset for optimum performances so please do not change any setting unless your instructor approves it. The digital camera has a video preview screen to facilitate previewing of the artifact
  • Make sure that the camera is level. This can be accomplished by adjusting the X and Y position and viewing the liquid level on the camera
  • As soon as all the photographs are complete, an NCPTT staff member will print them and disperse the finished photographs to each respective group.

At this time, please refer to to refer to Lab 1: Artifact Documentation Worksheet. [Download not found]

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