Object description
This 12th century hand warmer is made of two halves that can be unscrewed to access an iron cup which held extremely hot charcoal. The design is such that the cup remains upright regardless of its position. The heat was provided without scorching the holder’s hands due to the clever design. Indeed, it is better than the modern ones we have today as it is large enough to warm both hands at once.
These hand warmers were commonly used by priests during Mass in the Middle Ages. This is because increased care was given to the handling of the Sacrament. Therefore, there was less chance of the priests dropping the host or spilling the chalice due to cold or numb fingers!
The object is a functional one, yet we can see that it is also highly decorative. There is an intricate design of foliage with curling tendrils and leaves.

Decorative hand warmer/Metal,Copper Alloy,Bronze (cast)/12th century AD/The Hunt Collection/PD
Other types of hand warmers
Between the 14th and 18th centuries, hand warmers were also extremely popular in China. Made of copper rather than bronze, hand warmers in China would have been almost exclusively owned by upper-class families. Similar to the bronze hand warmer in the Hunt Collection, they contained a piece of charcoal embedded in ash to provide heat. From the 18th century, some Chinese hand warmers were also made of enamel and painted in a highly decorative fashion. Painted enamel was originally used for European-style objects but became popular in the Chinese domestic market.
In 18th century Britain, hand warmers began to be made from tin-glazed earthenware and often took the shape of a book. The one in the image below is in the Met Museum. It has an inscription on its side which reads: “The Gift is Small, Good Will is All”. Ceramic objects such as this were common from the 16th century and were intended as gifts or souvenirs.
It is interesting to note that an object with the same function could vary so much in design, depending on the intended recipient or market.

Hand warmer, China, 17th century. Copper alloy. Barbara and William Karatz Gift, 2019. The Met Museum.

Hand warmer, first half 18th century. British, Bristol. Tin-enameled earthenware; 5 1/8 x 3 1/4 in. (13 x 8.3 cm). The Metropolitan Museum of Art, New York, Rogers Fund, 1937 (37.123.3)
Sources
Dubansky, M. Book-Like Curiosities (Blooks) in The Met Collection. https://www.metmuseum.org/perspectives/blooks.
The Hunt Museum Essential Guide. Scala Publishers. 2002.