Introduction to slag analysis: What is it and why bother?

Following on from my introduction yesterday, I’m continuing my introduction to slag analysis with a quick and easy discussion of why we do it and what we hope to achieve.

What is slag?

Slag is what we call some of the remains generated by high temperature metallurgical activity. When people smelt ore into metal or work metal by heating it, the process produces left-overs, rubbish.

Ores often have unwanted minerals associated with them, like silica, and during the process of smelting this can combine with other materials in the furnace and the fuel to produce something that looks a lot like lava. This liquid rock or slag is unwanted, and is usually discarded.

When a metal is heated to high temperatures, it often oxidises. This oxide layer is often removed or falls off the metal, and sometimes it too can combine with fuel or other materials in the immediate area. It too is often referred to as slag.

The appearance of slag, which I will talk about later, depends on many factors and its chemical composition is also highly variable. However all slags share common origins in the production or working or metal.

Why do you want to analyse it?

Metallurgical activity has been undertaken by humans for thousands of years, but we know relatively little about it. Archaeometallurgists are interested in reconstructing the methods that past people used to produce metal, but we are often hampered by the lack of physical evidence. Most metallurgical activity made use of a furnace, a large structure containing a fire, but these structures rarely survive in the archaeological record. We rarely if ever find any examples of the tools past metal workers used. The objects themselves are traded, sold, moved about, used, lost, destroyed or disposed of. It is extremely rare to find any actual metal on the site of past metallurgical activity.

The one thing that survives well is slag. Metallurgical processes, particularly smelting, can produce large quantities of slag and it survives burial well. By examining this slag we can begin to answer questions that interest archaeologists and by extension, anyone interested in the site or people in the past:

•  What sort of metallurgical process took place on the site?
• What type of furnace or other structures might have been present on this site?
• What kind of metals and/or metal objects might have been made?
• How much was made?

From these questions we can start to ask questions about how the site fits in with our knowledge of the area, or the period or occupation. We can comment on how processes were undertaken on this site in comparison to others, and perhaps talk about where the products from this site were going and what they were for. How clearly you can answer these questions, if at all, depends on the quality of the evidence from the site, but that’s true of any archaeological excavation. If the evidence permits, there’s even chance of answering more widely reaching questions:

• On what scale were Iron Age people making iron in Britain?
• How did the Romans create brass?
• What really happens inside an iron smelting furnace?
• Was there ever a ‘Bronze Age’ in central Africa?
• Who were the first people to create cast iron?

These fundamental questions are the sort that really good sets of data can help to answer. In the past slag has been ignored due to its unattractive nature and sometimes inconveniently dense deposition, but since the 1960s there’s been a slow growth in slag analysis that has begun to answer some of the questions above, among others, but substantial areas of past metallurgical activity remain largely unexplored. By analysing slag, we try and improve our knowledge not just of individual sites and people, but of fundamental technologies that underpinned human development.

Next post in the series: How iron is reduced in a bloomery furnace.