Tablet Conservation Process

The processing of tablets prior to their final storage is a time-consuming, multi-step procedure.




  • After unpacking and initial checking of the tablets, the first step in their conservation and preservation is their entry into the CDLI FileMaker database. This allows for the easy retrieval of information about individual tablets and also permits sorting and organizing data on groups of related tablets.


     

  • Before tablets are baked they are first photographed. There is always the chance that baking a tablet may cause cracks or some surface damage, so it is vital to document their initial appearance. Sometimes the tablets are coated with ammmonium chloride before photography, but this is not always the case. The accompanying images show the heavily encrusted surface of a tablet before baking and soaking. Compare this image to the same tablet below after baking and soaking.

     

  • Tablets are baked for two related purposes: to preserve them and to clean them to improve their readability. Once fired to brick hardness tablets may also be more vigorously brushed to remove any remaining debris from their surfaces.

    A Fisher Scientific Isotemp® Programmable Muffle Furnace (650 Series) is used for baking the tablets. Up to 3 stackable ceramic shelves will fit within the furnace. Depending on the size of the tablets, up to 40 may be fired at one time.

    First the tablets are set to rise from room temperature (ca. 26ºC) to 150ºC at 0.4º/minute. It takes a bit over 5 hours to reach this temperature. The 150º is held for another 43 hours to allow all moisture within the tablets to evaporate and reduce chances of cracking or explosion due to steam build up within the tablets. Next the tablets are raised to 375º at 1.6º/minute over 2 hours and 20 minutes. A final increase takes the tablets to 675ºC at 1.4º/minute over 3 hours and 34 minutes. After this the furnace shuts down. The interior is cool enough to open within a couple days.

  • While tablets are in the ground, that is, before their removal from the soil, they are in a state of equilibrium with their environment. Once removed from the earth outside forces, such as heat and humidity, begin to act on them. Salts, and other minerals in the interior of the tablets, may be drawn to their surfaces that may be damaged and/or become unreadable as a result. Once tablets have been baked they become insoluble to water. This allows them to be soaked in one of the two sinks in order to dissolve these destructive salts and minerals. This improves the readability of a tablet's surface and removes the harmful salts and other minerals from the interior and exterior of the tablet. 
  • The lab contains two sinks which are used for soaking the tablets. A flexible plastic tube runs from the faucet down to the bottom of each sink. Once a sink is filled with warm water a trickle of warm water is allowed to continue feeding the basin; a drain near the top of the basin prevents any overflow. The warm water is more effective in dissolving the salts and minerals. The tubes are placed at the bottom of the sink to create circulation toward the drains above, which allows the water containing the dissolved minerals to be carried away.

    Square frames of PVC pipe supporting plastic mesh are used to hold the tablets while they are soaking. These frames may be stacked so that a large number of tablets may be soaked at one time, as many as ca. 80 per sink. The mesh allows the water to circulate freely through the sink. Tablets may be placed either directly on the mesh, or in small plastic dishes (especially if there is any danger of the tablets coming apart while soaking). Tablets are soaked for three to four weeks, or longer for particularly encrusted specimens.
  • At the end of the soaking process, tablets are cleaned by brushing to remove any excess residue left on the tablet surface. Depending on the state of preservation of the tablet and the type of materials encrusted on the tablet, tablets may also need to be soaked in distilled vinegar (acetic acid), for example to remove calcium carbonate salts deposited on the surface. After soaking in vinegar, tablets are placed back in the water bath for one day to remove any residual acidity. Upon removal from the water, tablets are dried on a counter covered with wax paper to prevent sticking. Prior to being photographed, tablets are brushed again with a soft-bristle brush; a metal dental pick is also used to remove encrusted soil from the tablet surface to maximize the readability of signs.
  • Tablets which have broken apart during the baking process or after having been soaked are glued back together using "Zap-A-Gap Medium CA+" glue. Although every effort is made to remove fake bits or to separate pieces from different tablets at this point, the repair process is reversible either by re-baking the tablet to vaporize the glue or by soaking the tablet in acetone, which softens the glue. Very fragile tablets are consolidated by applying "Zap CA Thin CA" to the tablet edges and to eroded areas which do not contain written signs.
  • The last stage before permanent storage of the tablets is final photography. This is usually, though not necessarily, done prior to final publication when scholars working on the tablets want to examine the tablets under the best possible conditions.

    The first step in final photography is coating the tablets with a fine layer of ammonium chloride. This gives the surface of the tablet a stark white appearance that highlights the wedges of the cuneiform script and any seal impressions.

    Granules of ammonium chloride are placed in a specially shaped glass tube designed by David I. Owen. The tube, on a stand, is held horizontally over a low flame. The back end of the glass tube is attached to a length of rubber surgical tubing, the other end is open. After the ammonimum chloride is heated and vaporized, air is blown through the tube and the ammonium chloride mist coats the surface of the tablet. It is vital that the surface of the tablet has a uniform, white coating in order to maximize the contrast between the surface and the writing and impressions. Care must be taken to coat only the parts of the tablet that will be photographed immediately because the ammonium chloride dissipates from the surface fairly quickly. This also reduces the chance of leaving distracting finger prints in the ammonium chloride. After photographs are completed the ammonium chloride can be removed by simple brushing or by allowing it to evaporate.
  • Once part of the surface of a tablet has been coated with ammonium chloride it is quickly photographed before the coating evaporates. At least two photographs of each side of a tablet under different lighting conidtions are initially made. If these are not satisfactory the tablet may be recoated and additional photographs taken.

    These digital images are then imported into PhotoShop© where the brightness and contrast are adjusted in order to enhance the differences between tablet surface and any text and/or impression(s). Compare this publication quality image of a tablet with that of its initial appearance above.