Abstract: An apparatus is provided which includes a holder operable to retain an article for interaction with a medium. The article has a first portion and a second portion, and the medium is such that the interaction alters the article in a temperature-dependent manner. First and second temperature-modifying elements are maintained by the holder adjacent to the first and second portions of the article to facilitate heat transfer between each temperature-modifying element and the adjacent portion of the article. The apparatus also includes a controller which is operable to maintain the first and second temperature-modifying elements at first and second independently controlled temperatures, respectively, such that the rate of interaction of the medium with each portion of the article is variable in a manner dependent upon the temperature of the adjacent temperature-modifying element.