York in 866, the Northumbrians, unsuccessfully, a year later. Nor had the preceding century of Northumbrian rule been peaceful; between 758 and 867, every King of Northumbria whose fate is known was either murdered, killed in battle or forced out. Any one of these turbulent periods could have inspired the owner of the helmet to hide it with the unrealised intention of recovering it later. Conservation For the five weeks following its discovery, the helmet was placed in an airtight Perspex box with a humid nitrogen atmosphere. This was to solve the seemingly contradictory problems of conserving any remaining organic matter, which would need to be kept moist, and conserving the iron of the helmet, which would normally need to be kept dry to avoid corrosion. The humid nitrogen environment avoided the latter danger by removing the oxygen needed for oxidation to occur. In this state the helmet was held stable; to allow for radiography and other examination it was removed from its container four times, for no more than two hours at a time, at which point some rusting occurred. The scans revealed the presence of the camail and the sinister cheek guard within the cap of the helmet, otherwise filled with clay. In mid-June the interior of the helmet was excavated in intervals, corresponding to the vertical slices taken of the helmet when it was CT scanned. No significant organic materials were found—it had been hoped that an interior leather cap, worn as additional padding, might be present—allowing the helmet to be protected against corrosion more easily, by sealing it in a new Perspex box desiccated with sachets of silica gel. The helmet next had the accumulated layers of corrosion removed. This was done manually, using brushes and a scalpel, with an eye towards preserving the corrosion which itself retained the original surface texture of the helmet. Micro-abrasive blasting was used on some areas such as the sinister cheek guard, after its broken fragments were adhered together, as the corrosion was too heavy and the surfaces too fragile to press against with the scalpel. Most of the brass fittings needed only cleaning with glass bristle brush, and the interior of the cap was only lightly cleaned, leaving material for possible future analysis. The mail, extremely well preserved despite being a cemented block when removed from the cap of the helmet, was freed by using a scalpel and mounted