When making dense fruit cakes such as a Christmas cake, the batter needs to be heavy enough for the dried fruit and nuts to be suspended in it; if it's too thin the fruit will sink to the bottom. Another challenge comes from the sweetness of the dried fruit, which will scorch and turn bitter if the oven temperature is too high. This is why traditional fruit cake recipes often require you to bake the cake slowly at a low temperature and to line the inside and outside of the tin with paper: a double thickness of parchment paper inside, and several layers of newspaper secured with string outside.
