A slow cooker allows any desired cooking time; the more energy-efficient vacuum flask must cook within the time taken for the food to cool below cooking temperature.
Early birds staked out key positions, bringing with them blankets, chairs, food and vacuum flasks to keep warm for the more than 10-hour wait to see the royal couple.
The vacuum flask was so efficient at keeping heat out that it was found possible to preserve the liquids for comparatively long periods, making examination of their optical properties possible.
