Metal pans filled with hot coals were used for smoothing fabrics in China in the 1st century BC. From the 17th century, "sadirons" or "sad irons" (from Middle English "sad", meaning "solid", used in modern English through the 1800s) began to be used. They were thick slabs of cast iron, triangular and with a handle, heated in a fire or on a stove. These were also called flat irons. A later design consisted of an iron box which could be filled with hot coals, which had to be periodically aerated by attaching a bellows. In Kerala in India, burning coconut shells were used instead of charcoal, as they have a similar heating capacity. This method is still in use as a backup device, since power outages are frequent. Other box irons had heated metal inserts instead of hot coals.