In the five years since the discovery of the Staffordshire Hoard, the more than 4,000 pieces of the hoard have been cleaned, cataloged and grouped by physical and stylistic similarities. Five hundred more objects and fragments were found hidden in the soil clumped on the pieces and in a follow-up 2012 excavation. About 1,000 new joins have been discovered, allowing conservators to puzzle together objects that have never been seen before, including previously unknown types of sword fittings and mounts. More than 1,500 pieces have been identified as fragments of a single helmet. The hoard has also been officially dated to the late 6th, early 7th century, an important transitional period between the decline of traditional Anglo-Saxon polytheism and the advent of Christianity.

Researchers have now turned to analysis of the composition of the alloys. Minute scrapings taken from the surface of about 200 gold objects were viewed through a scanning electron microscope exposing an ingenious system of making gold with a high silver content look as shiny as pure as the real thing.

The technique was not written down in Anglo-Saxon times, and had never been detected in metalwork from the period, but a similar technique was known from Roman accounts. It must have been spoken about by the brilliant Anglo-Saxon metal workers, and involved taking gold which was alloyed with up to 25% silver, and heating it in an acid solution – made from iron rich minerals such as brick dust – so that at the surface the silver leached out and could be burnished off. The surface would then appear to be the highest quality gold, but just below the surface there was inferior metal.

“They knew what they were doing,” said Eleanor Blakelock, the scientist who discovered their secret. “This wasn’t something which could possibly have happened by accident.”

The technique left the surface with more than 90% gold while just underneath it was 70 to 75% gold. The shiny surfaces could be enhanced with contrasting decoration like wire filigree made from darker, less pure gold. The objects intended for males — belt buckles, weapon fittings — appear to have a higher gold content than jewelry worn by women. Experts don’t believe it was done as a deception to pass off a cheaper alloy as gold. It was just a way to make the best of the materials at hand.

The discovery has also revealed new information about some of the objects. The five hilt fittings from a seax, a single-edged knife, for example, have an odd man out. Four of the pieces have similar alloy composition and were given the acid solution surface treatment. The fifth piece, the pommel cap, has a different alloy composition and was not given any surface treatment. That suggests it was a replacement piece or a later addition to the seax.

Other analytical methods archaeologists used to test the metal content of the surface are not able to detect this technique, which means 1,400 year-old Anglo-Saxon metalworkers have done an impressive number on 21st century technology. Archaeologists now know they can’t trust standard surface analysis to determine the gold content of an artifact, Anglo-Saxon or otherwise.

In related news, the Birmingham Museum & Art Gallery, co-owner of the hoard along with the Potteries Museum & Art Gallery, opened its new permanent Staffordshire Hoard Gallery on Friday. About 300 pieces are on display in an exhibition that covers how the objects were made, how they were used before being broken up and buried, the tools conservators employed to clean the hoard. There’s even a Mead Hall to give visitors a glimpse into the life of the kind of Anglo-Saxon lord who would have owned such expensive weapons.

History West Midlands has a wonderful collection of videos uploaded this year about the conservation of the Staffordshire Hoard and overviews of eight featured artifacts (subscribe to the YouTube channel for future videos) I dare you to watch just one.

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