A cold shelf is a flat sheet of metal that is designed to fit directly onto the runners in your oven (no need for a shelf beneath) and doubles as a baking sheet when used in combination with a sheet of Bake-O-Glide™.
A few myths about cold shelves:
‘it should be stored in the bottom of the oven and removed when needed’ - nope, keep it outside the oven where it will be cool!
‘it will cool down the rest of my oven’ - nope, it will shield things cooking below it temporarily - it doesn't reduce temperatures as such!
‘it will stay cold throughout my cooking’ - nope, the effect will only last 20-30 minutes! Grab a couple if you need to cook for longer.
Cold shelves are used to shield certain foods from the fierce heat of the roasting oven; it is this which allows the cold shelf to turn part of the roasting oven in 2-oven Aga range cookers into a pseudo baking oven.
The reason for the steep heat gradient between the ovens (and therefore the need in some instances to ‘shield’) is the use of radiant heat. Conventional fan-ovens create heat at the back of the unit and use a fan mechanism to keep the hot air circulating to heat the food; the Aga range cooker works by using a top-down approach to heat by distributing the stored energy held within the unit from the top of the roasting oven - which is why food placed high up in the roasting oven is browned very quickly!
To create a cooler environment this heat needs to be physically deflected by placing the cold shelf above the item thus shielding it from excessive heat; this technique can be used for between 20-30 minutes.