Write:
NBTTagList furnacesTagList = new NBTTagList();
        for (Map.Entry<BlockPos, BakingProduct> entry : furnaces.entrySet())
        {
            NBTTagCompound furnaceCompound = new NBTTagCompound();
            BlockPosUtil.writeToNBT(furnaceCompound, TAG_FURNACE_POS, entry.getKey());

            if(entry.getValue() != null)
            {
                entry.getValue().writeToNBT(furnaceCompound);
            }

            furnacesTagList.appendTag(furnaceCompound);
        }
        compound.setTag(TAG_FURNACES, furnacesTagList);


Read:

 final NBTTagList furnaceTagList = compound.getTagList(TAG_FURNACES, Constants.NBT.TAG_COMPOUND);
        for (int i = 0; i < furnaceTagList.tagCount(); ++i)
        {
            final NBTTagCompound furnaceCompound = furnaceTagList.getCompoundTagAt(i);
            final BlockPos pos = BlockPosUtil.readFromNBT(furnaceCompound, TAG_FURNACE_POS);
            final BakingProduct bakingProduct = BakingProduct.createFromNBT(furnaceCompound);
            furnaces.put(pos, bakingProduct);
        }