This century, researchers such as Roland Warzecha have worked to spread awareness that most round centergrip shields from the Baltic were rather thin in the centre and even thinner at the edges. An overall thickness, including wood, skins, and any intermediate layers, of about 8 mm in the centre and 4 mm at the edge is typical from the sacrifices at Illerup Ådal around 200 CE to late Viking Era graves around 1000 CE. We do not know as much about shields in dryer, warmer parts of Europe where it was not customary to deposit arms in lakes and bogs. But we can study the surviving shields from La Tène on Lake Neuchâtel in Switzerland. If you read the original reports, you will learn many things which Peter Connolly did not tell you.
Paul Vouga wrote about the two shields from La Tène twice, once in his annual reports and once in his summary of the excavations. These shields differ from later plank shields in that they do not yet have a round iron boss covering the handhole. Instead, they have a long wooden ‘spine’ (Latin spina) shaped like a spindle which is usually reinforced over the handgrip with a sheet of copper alloy. Archaeologists call this reinforcement the umbo although that word also refers to a metal boss. The shields have a short horizontal handgrip reinforced with iron instead of a long vertical handgrip like Viking shields.
One shield consisted of a central plank 28 cm wide with a curved top and bottom, and two shorter planks with one curved side and one straight side, It was 11 mm thick in the centre, 8 mm thick at the middle of the sides of the central plank, and only 3 mm thick along the curved edges of the planks. It was discovered as three separate planks. The other shield seems to have been made from a single piece of wood. It had about the same thickness in the centre and along the rim as the three-plank shield (Paul Vouga, La Tène, Monographie de la station publiée au nom de la commission des fouilles de La Tène (Leipzig 1923) https://www.academia.edu/66964016/Paul_Vouga_La_T%C3%A8ne columns 59-62).
For every surviving shield we have hundreds or thousands of sets of iron and copper-alloy fittings for shields. Because they were attached with rivets which were peened or clenched over onto the back of the shield board, archaeologists can study how thick the shield was where the metal was attached.
Les rivets de fixation de l’umbo présentent, sous les ailettes comme sous la coque des bivalves, un repli à angle droit, caractéristique de l’épaisseur de bois qu’ils traversent. Dans le cas présent, l’épaisseur minimale est voisine de 10 mm. C’est la plus fréquente. Les épaisseurs supérieures, entre 14 et 16 mm sont assez rares. Dans les deux cas, l’épaisseur du bord de la planche ou de la structure composite qui en tient lieu s’amincit au moins de moitié si l’on en juge par l’ouverture de l’orle métallique qui protège ce rebord (cf. planche XLI).
The umbo is fastened with rivets on the wings and on the hull of the bivalve. These are bent at a right angle, indicating the thickness of the wood which they pierced. In the current case, the minimum thickness is about 10 mm. This is also the most frequent thickness. The greater thicknesses, between 14 and 16 mm, are relatively rare. In both cases, the thickness of the shield board or of the composite structure was thinned by more than half, if one may judge by the opening of the metallic rim which protected its edge.Jean-Louis Brunaux and André Rapin, Gournay II: Boucliers et Lances, Dépôts et Trophées (Éditions Errance: Paris, 1988) p. 32
So the tradition of making plank shields thick in the center and thin at the edges was not limited to the Baltic and North Sea region or to round shields. Surviving European shields from 1180 onwards are not aggressively thinned between the center and edge. These later cavalry shields must have been intended to be used differently than earlier infantry shields, because they are stronger but heavier for a given size.
Vouga was puzzled by his findings:
The thickness of the board- 11 mm in the centre and 3 mm at the rim- leads to the conclusion that the weapon would not have been usable in the condition in which it has come down to us: the least blow would have split it!
It could not have been reinforced with metal plaques, which would have been cumbersome and would have been preserved- nor, in our opinion, with leather, traces of which we would have found under the umbo, or upon the shield itself, since we possess substantial fragments of a sack of leather which had served as a bundle. Raw hide would have simply decayed without leaving traces, but we believe that as it contracted it would have caused the delicate rim to burst apart, or in any case, the rim would have been crumpled by the pressure.
Yet, as the excavations of 1915 brought to light, in the immediate vicinity of the shield, a fragment of board with a thickness of 10 mm to which a fragment of plant-based fabric was stuck, as in addition, we have met on many occasions other fragments of plant-based fabric, notably of sacks en paille tressés à double, … we ask ourselves today whether these finds of sack, … could not be traces of the reinforcement of the shield. Armoured in this way, the arm would have remained light- an essential requirement- without, however, offering resistance to the Roman shield, which explains the statement of Caesar (de Bello Gallico i.25) that “the shields of the Helvetii were pierced and nailed together by the Roman javelins.”Vouga columns. 61, 62
Today we know by experience that thin wooden shields can easily be covered with rawhide, although they tend to bend into a ‘potato chip’ shape as the wet rawhide dries. Covering both sides can equalize the pressure and reduce the distortion. But rawhide, tanned leather, linen cloth, and hemp cloth have all been used to cover wooden shields.
Anonymous cowards on the Internet are snarky that Vouga did not record some things which they wished to know. But he recorded many things which are important, and which archaeologists today do not always see and write down.
Edit 2022-06-18: added a better citation to Vouga, clarified that one-plank shield was similar in thickness to three-plank shield