If you are shopping for a new bed these days, you have the option of choosing from a number of standard sizes including twin, full, queen and king. Antique lovers do not always have the same advantage. Although beds constructed after the late 1800s were generally standard in size--either twin or full--larger beds are rare and where they do exist often require the use of custom-made bedding made to non-standard sizes. Fortunately, there are solutions for those who would like to decorate their bedrooms with fine antique double beds while still enjoying the comfort and space of a queen-sized mattress.
Remove the side rails and cross slats from your double bed, leaving only the headboard and footboard. Use a standard, inexpensive converter kit that can be purchased on-line or at most bedding stores.
Store the original side rails and cross slats in the event you later want to reduce the size of the bed or to sell your antique in its original configuration. Standard converter kits resemble modern metal bed frames. They consisted of an L-shaped steel rail system which, when assembled, supports the mattress without the need for cross-slats. Conversion kits come equipped with hardware to allow you to attach the new frame to the headboard and footboard.
To attach a headboard-only modern queen bed frame to an antique headboard and footboard, purchase adapter conversion plates which clamp on to the existing steel frame. Bolt the headboard and footboard into these plates that are widely available on-line and can be found in many specialty hardware stores and sleep shops.
To put a queen size mattress on a double sized iron bed with a frame that is welded onto the headboard and footboard, install a set of 4 L-shaped brackets to the metal side rails, allowing a mattress to be seated atop the frame without shifting from side-to-side.