Cool stuff! I have seen this method used in creating "barn wood" furniture, basically taking new wood and making it look grey and oxidized like barn wood. Some wood species really lack tannins (like pine) and the reaction won't work well (hence the need for strong black tea). Other species like oak will turn almost black without the use of tea. Although if you are going for a jet black look your best bet is to use strong tea no matter what kind of wood you are working with just to be safe.