Tubular open-stitch knitting of the previously described type is a common find at Viking sites. This discussion began with it because there is little question about it being stocking stitch in the present-day Western sense, albeit with a compound structure. Comparable specimens with twisted stitches have also been found, as has the cross-knit looping that can often be difficult to distinguish from it.
Correctly differentiating the knitted and looped forms is not eased by their often being conflated in the literature of the modern craft known as “Viking knitting.” Despite calling this technique knitting, it is straightforward cross-knit looping. The underlying issue is that twisted-stitch knitting and cross-knit looping share the same basic structure. An archaeologically-recovered metal tube made in either manner may not reveal the secondary structural detail needed to tell them apart, especially if only a fragment remains. Other indications of the production method might also help but we lack specific knowledge of the techniques used by the Vikings to make any such wirework. It is possible nonetheless, to test the suitability of more recent methods and list conceivable options.
The post linked to above cites a remark by Richard Rutt about the equivalent Irish tubular knitting: “…the Celtic work was done with a knitting nancy. Complex knitting is much easier on a knitting nancy than it is on knitting needles.” (A knitting nancy is a small peg loom, also called a knitting spool.) This judgment would be entirely correct for work with yarn and is consistent with what other authors have suggested about the use of a peg loom for the Egyptian knitted tubes. However, metal has a number of mechanical properties that yarn does not. It is vulnerable to kinks and dents, breaking easily at either, and hardens when worked. All of the methods for open-loop knitting on a peg loom put the stitches under significant tension, as does compound knitting. This can become critical with wire, particularly at small diameters. The method that strains it the least, minimizes the risk of damage.
The stitches on a peg loom are commonly worked with a hook. Although a peg loom can be used for knitting both yarn and wire, a hook alone is adequate for the latter. Wire is rigid enough for a loop formed of it not to require mechanical support pending its being secured to another loop. The process is illustrated in this video where it is named for the tool, but the parent website also uses the more analytical designation “Invisible Spool Knitting” — ISK.
This basic technique will be familiar to any knitter who has repaired dropped stitches by re-knitting them vertically with a crochet hook. It is easy enough to see how it can also produce a twisted-stitch structure simply by rotating each loop 180° after it is pulled through the corresponding loop in the previous row. If one similarly envisions a loop of the one color wire being drawn over the loop of the other color in the next row, and then secured to the loop of its own color in the row after that, the result is compound knitting. (The strip of 8th-century Irish flat knitting shown in the preceding post displays precisely such a two-colored compound structure.)
The drawplate is another important implement specific to wirework. It allows a tube to be knitted at a comfortably large diameter which is then reduced to whatever is desired for the finished tube. This means, at least in principle, that knitting needles can be used to form a looser version of the same structure, which is then drawn down to size — but also elongated in the doing. Ensuring adequately dense stitching in the final tube is one of the effects of compound knitting. It also appears for that purpose in the tutorial literature of modern Viking knitting as double knitting, with triple knitting also described.
Again, though, none of this establishes how the Viking and Irish tubes were actually knitted. Nonetheless, of all the plausible alternatives, using a hook in the manner shown in the video affords the greatest economy of motion and subjects the wire to the least stress. Minimizing the number of contact points that can potentially injure the surface of the wire might similarly explain a preference for an open-loop structure.