If you need to attach two slabs together and you clay is especially on the drier side, you will need slip. Water on its own can cause your clay, when drying, to actually crack apart. Imagine the dry desert, the reason there are so many cracks is because dry clay swells when it gets wet and clay that is too wet looses its ability to stay together. If you use a medium like vinegar, it causes the clay particles to slip together, not swell. So, take my advice and use slip. Also, only dampen your work, do not soak it ever. This will quickly show you how to make slip with both wet and dry clay using vinegar.