Dealing with weight and density of ceramic vessels and sherds

Volume and capacity have a relationship with weight. This is especially true for two separate ways of using such measurements:

  • calculating the ratio between the weight of the container (either amphora or other container) and the weight of the liquid stored inside;
  • using the weight of potsherds as a quantification method.

Suppose you have a complete profile of a vessel, in the form of a drawing. As often happens, only part of the vessel is preserved. Nevertheless, you would like to calculate what is the proportion between the total vessel and the sherds that were found in a specific archaeological context.

This is possible, and not difficult. The envisaged procedure should be as follows:

  1. calculate the total volume of the vessel from the drawing – note that this means the volume of the ceramic body and not the capacity of the vessel
  2. from the volume, obtain the total weight through density
  3. weight your sherds and see the proportion

Knowing the density of a ceramic body might not be immediate, and it’s not certainly a good idea to stick with a standard value good for contemporary ceramics (that is around 2 kg/dm³, by the way).

Measuring the density of a ceramic body

The density and porosity of a ceramic body can be obtained through Archimedes’ principle. A formal procedure is described in [ASTM-C373] (not publicly available, unfortunately), and it needs only water and a weight scale. Here follows a basic summary of the procedure.

[ASTM-C373]ASTM Standard C373 - 88(2006) “Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity and Apparent Specific Gravity of Fired Whiteware Products”, ASTM International, West Conshohocken, PA, 2006. DOI: 10.1520/C0373-88R06, http://www.astm.org/

There are three measurements that need to be taken: - dry weight \(W1\) - weight of the ceramic body in water \(W2\) - weight of the water-saturated ceramic body \(W3\)

From these three values, the following properties can be obtained with simple formulas:

  • external volume \(V = W3 – W2\)
  • bulk density \(bD = W1/V\)
  • apparent density \(aD = W1/(W1- W2)\)
  • apparent porosity \(P = (W3 - W1)/V\)
  • water absorption \(A = (W3 -W1)/W1\)

Note

This section was based on content published by Antonio Licciulli on his faculty website. You can find detailed content, mostly in Italian, at this page

A basic example

What follows is a basic procedure that was tested on a limited number of samples. You may want to use a spredsheet software to help with the recording and the quick calculation of the properties derived from the weight measurements.

You will need a container for water that is both wide enough to put ceramic sherds inside it and tall enough to have them stay completely below the water level. You will also need to keep the ceramic body suspended in water, almost certainly in vertical position.

Step by step:

  • weight the dry sherd (\(W1\));
  • put the sherd inside the water, and let it stand on the bottom of the container until you see no air coming out of it: the ceramic body is now saturated with water;
  • when the ceramic sherd is water-saturated, move the container with water (and the sherd on the bottom) on the weight scale and measure the weight \(p1\)
  • use a pair of pliers to lift the ceramic sherd from the bottom, and keep it suspended in water and completely below the water level – taking care that the pliers will stay as much as possible out of the water – and measure weight \(p2\);
  • take the ceramic sherd out of the water, let it quickly drip inside the container and weight the container with only water (the sherd should now be out of the container, on the desk), recording \(p3\)
  • optionally weight the ceramic sherd you just took out from water, recording \(p4\).

At this point, the dry weight is:

\[W1\]

while the weight of the ceramic body in water is:

\[W2 = p2 - p3\]

and the weight of the water-saturated ceramic body is:

\[W3 = p1 - p3 = p4\]
\(p1\) must always be the largest measurement, and \(p3\) the
smallest one:
\[p1 > p2 > p3\]

All the required measurements are now available to calculate the density, porosity and water absorption of the ceramic body.