An Expert Visit: identifying production techniques

Our project relies upon collaboration with a number of different professionals whose expertise we are lucky to benefit from. Canterbury jeweller Justin Richardson recently joined us up at the Petrie to view the instruments first-hand; Justin will be hand-making replicas of some of our selected metal bells and cymbals using authentic materials and techniques, so he needed to identify the methods of manufacture used to create the originals. Many of the processes used in Roman Egypt are still employed by metalworkers today, and looking closely at the artefacts can reveal tell-tale marks on the surface of the metal left from specific manufacturing techniques.

Cymbal with pitted surface. Photo: Ellen Swift.

The first thing Justin noticed was that some of the cymbals have a distinctive pitted surface in places. This texture was created by the ‘sand casting’ method of manufacture. This process involves creating a mould by pressing each side of the object to be copied into damp sand, thereby leaving a perfect imprint. Molten metal is then poured into the cavity, leaving a distinctive mottled surface in the process. Sand casting requires an original version from which the mould must be made; this was likely forged by hand, with any hammer marks on the original obliterated by the sand casting process rather than being transferred onto the copies. Sand casting is a cheap and easy method of production, and produces a lower quality object than ‘lost-wax casting’ – making it ideal for an object as simple as a small metal cymbal.

Hammered and lathe-turned cymbal, with caulked edge. Photo: Petrie Museum.

In contrast, the surface of another one of the cymbals revealed hammer marks. It was also significantly lighter than the others, leading Justin to conclude the object had been made by hammering a flat disc of metal over a hemispherical mould to create a dome shape. He also identified a caulked edge – caulking is a metal working technique where the edge of a piece of metal is hammered, end-on, to thicken it up and make it appear more robust than the item actually is. Further identifiable traces included concentric circles on the surface along with a perfectly centred hole, revealing further work on a lathe. Justin’s technical knowledge also revealed some unexpected surprises. The two main copper alloys used for the instruments are brass and bronze; our modern understanding of these materials and the kinds of objects they make resulted in us expecting any brass objects to be handmade,  and bronze objects to have been cast – however the exact opposite of this appears to be the case! The manufacture of a bronze bracelet with a bell attached also caught us out.

Cast bronze bracelet with dot decoration (UC58537). Photo: Ellen Swift.

It would seem logical to make such a flat bracelet by cutting out a strip of metal from a sheet, and then bending it to shape, however closer inspection revealed the object to be cast; the decorative motifs appear soft edged rather than cut into the surface of the metal, and the indented dots revealed no ‘ghosts’ on the reverse. Ghosts are marks left on the reverse of a metal surface when tools are hit against it to produce decorative marks. However, as no such ghosts exist on the bracelet, it again supports the case that it must have been cast. Whilst unexpected, casting such bracelets makes sense as it allows mass production of identical objects with little effort or additional expense, additionally suggesting this bracelet was not a one-off, but in fact one of a batch produced.

We learned a lot from Justin’s visit that we can use when looking at other metal objects in the Petrie collection. We look forward to seeing the replicas and will post an update on them soon. If you’d like to know more about Justin’s work, you can find his website here.



Laser Scanning 1

As well as compositional analysis, a key element of our project is the laser scanning of our selected range of musical instruments from Roman Egypt. The Petrie has some amazing examples of instruments that struggle to survive outside of the arid Egyptian environmental conditions – we have wooden clappers, metal bells, reed flutes and panpipes. Laser scanning these items allows the recreations of these objects, either through 3D printing or through the making of replicas using authentic materials and techniques. This will allow the instruments to be played, giving us the chance to hear the music of Roman Egypt in the 21st century.

A copper-alloy bell on a child’s bracelet from the Petrie Museum; the thinness of the top section of the suspension ring indicates heavy wear, presumably over many years. Photo: Ellen Swift.

The Classical & Archaeological studies department at Kent is equipped with a laser scanner and our technician Lloyd Bosworth joined us up at the Petrie for a day of scanning. The focus of the day’s activities were the Roman copper alloy bells and cymbals from the collection. The scanner sits on top of its own freestanding tripod, and has a fully articulated arm on which the scanning gun sits. This means you can scan objects on a table by pulling on the trigger and moving something similar to a barcode scanner over the artefact. Simple really. Or at least Lloyd makes it look simple – after having a go myself it’s clear that it requires a lot of hand eye co-ordination in order to keep the scanning gun the correct distance from the object, whilst also capturing every surface of the artefact.

Lloyd scanning one of the bells from the Petrie collection. Photo: Jo Stoner


However, once I stepped aside it was a successful day, and Lloyd scanned all of our chosen objects. These included a round bell similar to modern sleigh bells – a particular challenge was trying to scan the gap in the surface where the ball-shaped clapper was inserted. Our selection also includes a number of small bells attached to tiny bracelets, which we think were worn by children as an amuletic device (and also perhaps to keep tabs on where they were!). We also scanned a fantastic bell in the shape of the head of Bes, the dwarf god of ancient Egypt who protected mothers and children, and whose worship continued into the Roman period.

Roman bell in the shape of the Egyptian god Bes. Photo: Ellen Swift.

Once all the scan data is processed, the digital models of the instruments can be passed to our craft practitioners who can use the highly accurate measurements and surface details to create our replicas. We’ll update again soon on our progress!

Bells, Cymbals, and Radiation

An important element of reconstructing musical instruments is making sure they are made out of the right stuff. In making our replicas of the bells, cymbals, pipes, and clappers in the Petrie Museum, we need to make sure we are using materials that are as close to those of the originals as possible – this gives us the best chance of ensuring that they will make the same sounds as the original artefacts. For example, the kind of alloy used in the cymbals will affect the resonance and tone of the sound they create when struck. So it is important to get it right! This is where compositional analysis (and a fair bit of science) comes in.

One of the cymbals selected for XRF analysis. Photo: Ellen Swift.

We have been using an XRF machine to find out exactly what our metal musical instruments are made of. The XRF (“X-Ray Fluorescence”) machine uses radiation to identify the elements present in an object; the x-rays cause the artefacts to emit radiation that is characteristic of its own elemental makeup, allowing the equipment to identify the component parts (if anyone wants a more scientific explanation, see here!). In the case of our metal artefacts, the XRF tells us exactly what the ratio of different metals were in each alloy used. All the bells and cymbals seemed to be variations of copper alloy, either towards brass (an alloy of copper and zinc) or bronze (an alloy of copper and tin) – however the compositional analysis has ensured we know exactly the ratios of these, and therefore can choose the most suitable modern alloy for the replicas.

The XRF machine in action at the Petrie. Photo: Jo Stoner.

The XRF machine is a fairly hefty bit of kit, thanks to the lead lined case that it comes with – this contains the object when exposed to the X-ray, and keeps us safe from the radiation. It is however portable enough for it to come up to the Petrie for a day’s worth of compositional analysis. To get accurate results, the objects need to have a clean surface – the artefacts in the Petrie are great for this as, by and large, they were thoroughly cleaned and conserved post-excavation, preserving their surface. Thus many of our readings were accurate enough to identify traces of gold, as well as the more usual bronze, brass, and iron, which suggests the presence of gilded surfaces now deteriorated and no longer visible to the eye.

The compositional analysis of this bell showed that it was originally gilded. Photo: Ellen Swift.

Now we have this data, we can undertake the next step – meeting with our replica makers to discuss the most appropriate alloys to use, as well as which manufacturing techniques should be used to ensure their authenticity.