To get a piece of wood to sink you need to expel the air and replace it with water. I learnt how to do this first time I had wood to sink. First, boil the wood for an hour. Prepare a bucket of cold water. The colder the better. Very carefully get the hot wood and plunge it into the cold water. Weigh it down. Leave it there until it cools down. Then repeat the process of boiling and cooling, until it sinks. From my understanding the wood and air expands when heated so the air escapes. When the wood is dunked into cold water it contracts and sucks in the water. Each time you do this air is expelled from deeper and deeper in the wood and replaced with water.