4 DISCUSSION

The tests indicate that the tack is caused by the nature of Regalrez 1094 and not by the solvents or other additions. A recent product data sheet issued by Hercules gives Regalrez 1094 a Tg of 33�C (Hercules 1998c). The Tg measured by de la Rie and McGlinchey (1990, 168) is 43.8�C. The difference may be due to the methods used or to a change in the product. As we found, 33�C is also the temperature at which Regalrez 1094 gets tacky. Nevertheless the Tg alone is not enough to explain why a polymer is sticky at a certain temperature. Long molecules, intermolecular forces such as hydrogen bonds, and cross-linking are among the most important reasons many polymers with an even lower Tg are not sticky at 33�C (Horie 1994). Conversely, Regalrez 1094 has small molecules and few intermolecular forces, and it does not cross-link. These characteristics probably explain its tackiness at a relatively low temperature.

A varnish that gets tacky around 33�C might not be a serious drawback for paintings if the varnish is not touched and if the painting is kept in a climatecontrolled room. On the other hand, Horie (1994, 18-20) states that the effective Tg is much lower when measured over a period of one year or more. This finding means that dust pickup could result even with temperatures below 33�C. Dust pickup is actively taking place on the bed discussed at the beginning of this article, even though the temperature in the room has never reached 33�C. From these observations it seems that Regalrez 1094 is not a suitable top finish. To benefit from the good qualities of Regalrez, the use of the higher grade of Regalrez 1126 is discussed below.