Potting is an old technique derived from that of the medieval pie, which was often used deliberately as a way of conserving food rather than merely presenting it. The crust of the most solid raised pies was made from coarse flour. It was not intended to be eaten, except as a remnant thrown to the servants, but constituted a durable and airtight shell. The food inside it kept for quite a long time because, having been cooked in a sealed container, it was sterile. The explanation was not understood in the Middle Ages, but the effect was obvious in practice. Observation must also have shown that the weak point where decay began was the top of the filling which did not intimately touch the top crust, because the filling shrank in cooking; and which was exposed to the entry of air through places in the crust where steam had escaped. Therefore the top of the filling was sealed by pouring in, through a hole in the crust, boiling stock or melted fat, which luckily had themselves been sterilized by heating. When this addition solidified it sealed the filling effectively. Such pies were used, for example, for sending Severn lampreys from Gloucester to London, a journey of several days which could scarcely have been undertaken with any other of the primitive preservation techniques of the time.