After being gathered, the shells were then transported to the site of Clos des Châtaigniers. Transport may have either been by boat, given that the River Orne was navigable during protohistory, or by land. The return trip would have taken at least a half-day. The large number of barnacles (5,927 specimens) and small shells found in shell deposits seem to indicate that the preparation of the shells was probably done on the site. The barnacles were probably attached to the mussels and transported with them. It is likely that some of the barnacles detached from the mussels during the preparation or preservation phases. Traces of burning are evident on some mussels, representing on average 87% of NISP of the mussels. These marks are perhaps related to cooking methods or secondary activities occurring after consumption. Archaeological studies of burnt shell remains are not numerous in the archaeological literature. For this reason, we undertook research on the methods of cooking mussels and their effects on the shell, as described in the ethnohistorical literature, in order to gain some information (Waselkov 1987). Mussels can be cooked by placing them in the heart of a fire, in a steam oven (Best 1924, 417; Greengo 1952, 77; Duguet 1995, 367; Kroeber and Barrett 1960, 113; Meehan 1977, 366; Waselkov 1987, 101-2), placing them on hot stones (Terrell 1967, 44; Oberg 1973, 67) or roasting them around fires (Waselkov 1987, 101). Other hypotheses could also explain the high rate of burned archaeological mussels. After the mussels had been eaten, the shells could have been thrown into a fire. The shells could also have been used eventually to maintain the fire, or even to put it out. This activity could reflect hygienic and/or marine waste management strategies. Nevertheless, it is difficult to select one of these possibilities given the lack of available bibliographical references. However, experimentation on shell material could make it possible to identify specific criteria for each type of cooking method, as has already been undertaken for bones (Costamagno et al. 2010; Lebon 2010; Zazzo 2010). The results obtained would provide some real answers regarding cooking methods and heat treatments and seems a promising area for future studies.
The collected shellfish were either eaten directly or processed for later consumption. There are several different ways to preserve mussel flesh: it can be dried in the sun (with or without the shell), smoked and stored in jars, in baskets or strung on suspension lines and later rehydrated and boiled (Gifford 1939, 315; Greengo 1952, 77-78; Stewart 1943, 60; Kroeber and Barrett 1960, 113; Oberg 1973, 67; Aschmann 1975, 46; Best 1924, 417; Waselkov 1987, 106-7).
Previous research demonstrated, through experimental archaeology, that it was possible to dry a very large number of mussels in a relatively short period of time, with minimum effort, and using simple technology (Henshilwood et al. 1994). The most effective experiments in terms of time/calories related to the cooking and drying of mussel flesh inside the shell. Once cooked and dried, the flesh can be easily removed from the shell in only two or three seconds, so that fewer people and less time are needed to process large quantities of mussels. This method also reduces the need for firewood, since the mussels are cooked within around seven minutes. This technique eliminates the need for drying frames or posts and suspended lines, and minimises labour input (Henshilwood et al. 1994).
Finally, the dehydrated shellfish were generally later plunged into fresh water, and then boiled (Gifford 1939, 315; Swanton 1946, 378; Greengo 1952, 77-78; Kroeber and Barrett 1960, 113). For example, some North American populations living close to the sea in temperate areas kept dried shellfish all year round, either for exchange or for their own consumption (Greengo 1952, 78-80; Oberg 1973, 67-75). Nevertheless, we lack the archaeological data (organic residues in pottery, artefacts linked with preservation such as baskets, etc.) to state definitely that such a preservation technique was used at the site of Clos des Châtaigniers.