Abstract: An assembly which comprises a heat-recoverable article which is adapted to join substrates together and a plug which is in a recess of the article. The plug comprises a first polymer, e.g. nylon, which is opaque at room temperature and transparent at an elevated temperature, and preferably comprises a colored portion beneath the first polymer. When the article is connected to a power supply, it heats and recovers. At the recovery temperature of the substrate, the nylon melts and the colored portion becomes visible, thus indicating to an operator that the article should be disconnected from the power supply.