משנה אויף תהילים 19:5

A curved horn is susceptible to impurity but a straight one is clean. If its mouthpiece was covered with metal it is unclean. If its broad side [is covered with metal]: Rabbi Tarfon says it is susceptible to impurity But the sages say it is clean. While they are joined together the whole is susceptible to impurity. Similarly: the branches of a candlestick are clean. And the cups and the base are susceptible to impurity, But while they are joined together the whole is susceptible to impurity.