Like most newspaper items on esoteric subjects, there is often a bit (sometimes quite a bit
) of error introduced between the interview and the page, so don’t be surprised the facts seem ‘off’ in the Guardian article.
The Viking period spans a few hundred years, and the era includes some pretty radical improvements both in sword design and in steel making, so it would be a mistake to think there was just one way to make a sword during that era. Plus, swords made locally vs. swords made in an ‘industrial’ center & exported will add a layer of difference.
You can get more info on the fake issue and start forming your own theories on which ones were ‘fake’ from reading “The Vlfberht sword blades reevaluated”
by Anne Stalsberg, she did a signature analysis of ~135 Ulfberhts, and there are two variants of the signature (the two most numerous) she concludes are probably authentic.
Below is a Williams quote from 2003, when he was starting to develop this concept - note he is saying higher slag content (not quenching!) can lead to brittleness:
Some early medieval swords in the Wallace Collection and elsewhere
David Edge, Alan Williams
Gladius XXIII, 2003 pp. 191-210
It should be observed that yet another blade with a similar inscription has been found by one of the authors to consist of a totally different metal. That «Ulfbehrt» sword was made of an air-cooled hypereutectoid steel of around 1.0%C (Williams, 1977). Since that account was published, a great deal more information has become available about the crucible steel industry of Central Asia (e.g. Craddock, 1995 and Feuerbach, 1997) and it seems likely that a cake of such a steel was the raw material for that blade; being virtually slag free and of hardness around 300 VPH, it must have been an exceptionally serviceable sword, and one which would keep its hard edges permanently. The maker of our «Ulfbehrt» sword had made what must have seemed to his customers at the time like a very good copy, with an edge hardness of over 460 VPH. Prolonged use might have altered their opinions; the cutting edge is only 6mm deep, and could have been removed by a few years of regular sharpening on a grindstone. It is also distinctly higher in slag content, and therefore more likely to fracture on impact.