3.1.2 Interpreting a graph

Figure 11(a) shows a graph showing total global lead production. The horizontal axis is time and it ranges from a minimum of 5000 years before present on the left to the present day on the right; the scale is non-linear. The vertical axis is lead production in tonnes per year in a logarithmic scale and ranges from 0 at its minimum to 106 tonnes per year at its maximum. At 5000 years before present the production of lead was first discovered and production is at the minimum. This is labelled as ‘discovery of cupellation’ on the graph. A second point on the graph, around 2700 years before present, is labelled ‘use of coinage’ for which the lean production is between 102 and 103 tonnes per year. This point coincides with the start of the period labelled ‘rise and fall of Athens’ which lasts until about 2300 years before present. There is a peak of lead production around 2000 years before present at around 105 tonnes per year. After this peak, at about 1800 years before present, the graph is labelled ‘exhaustion of Roman lead mines’. Lead production then falls relatively slowly to 1000 years before present at around 104 tonnes per year, before starting to rise again. At about 1000 years before present, the graph is labelled ‘silver production in Germany’. Lead production rises slowly at first until about 300 years before present, when about 6x104 tonnes per year were made. This point is labelled ‘Spanish production of silver in the New World’. After this, the production of lead increases rapidly to more than 106 tonnes per year at present time. Figure 11(b) shows a graph showing the lead concentration in a Greenland ice core. The horizontal axis is time and it ranges from a minimum of 7760 years before present on the left to the present day on the right. The horizontal scale is non-linear although the times from 3000 years BP to present line up with the graph in (a). The vertical axis is lead concentration in grams per gram of ice core sample and ranges from less than 1x10−12 at its minimum to 4x10−12 grams per gram of sample at its maximum. There is a very low lead level of less than 1x10−12 grams per gram of sample up until 3000 years ago. The concentration then rises rapidly to a maximum of more than 3x10−12 grams per gram of sample around 2000 years before present which coincides with the exhaustion of the Roman lead mines. The lead concentration then falls back to very low values about 1800 years before present. The lead concentration then rises to reach a maximum of about 4x10−12 grams per gram of sample about 500 years before present.