If you just press two smooth pieces of clay together, they might stick for a while, but the connection can be a lie. For example, if you are adding a handle, a cup base, or a raised detail to a piece and then smooth the edge with your wet finger, from the outside it may look perfectly joined. But when your work shrinks during drying and firing, the new part will shrink too, and it may pull slightly away. A fine line opens in the seam, or it falls off while drying and shrinking. That is the purpose of scoring and slip.
Scoring is the act of roughening both surfaces that will be pressed together. If you are just pressing smooth clay against smooth clay, it is hard to connect them properly. Instead, use a tool, a small tooth or fork, or a needle tool to scratch shallow cross marks in the spots that will be joined. You don’t need to make deep scratches that will cut into the strength of your form, but you need enough texture to hold your slip. Think of scoring as a rough area. A join needs something to hold onto when you are building with handles, feet, or other raised areas such as coils or decorations.
Slip is clay mixed with water to make a soft creamy mixture. It is not water alone and it is not a glaze. Because it is a very thin mixture of clay, the slip will help connect the scored surfaces as you press them together. It will fill in your marks and create a good clay-to-clay bond when they touch together. Usually, a small amount of slip works better than a thick, messy layer. Too much slip makes the connection move around and the join becomes weak. The result should be a good moist connection of clay, not a puddle of slip between the parts.
The best way to see this for yourself is to make a quick test before joining any part of your form. Make a small clay tile and a clay strip. Scratch the end of the clay strip and the tile where you are going to join them together with a fork or tool. Use some slip, press the clay strip and clay tile together, then use a rib tool or your fingertip to gently blend the outside edge. Then do the same thing but this time do not scratch the edges. Use only water and smooth clay pieces together. Make sure both of them are firm enough to move. You may be able to see and feel the difference even before they are fully dry. The join will be tighter and connected with the prepared clay but it may pull away and show a dark line where the smooth join meets the surface.
The amount of pressure you apply is just as important as the materials. After you score both surfaces and add your slip, press the parts together hard enough to close the space, but not hard enough that you distort or crush the form. If you are attaching a handle, use one hand to support the inside of the form or vessel as you press from the outside. When you are joining a foot ring, check the clay strength of the base and see if it can hold up to this pressure. Attaching parts to the clay form should be a two-handed job, where one hand is protecting the form and the other hand is making the attachment secure.
The wetness of the clay affects how strong a connection will be when joining. Very wet clay can be difficult to add to a form if it slumps when you apply it. On the other hand, clay that is too dry may be hard to attach with just your slip. Sometimes, joining to clay that is softer and leather-hard is easier to work with, especially when the form holds its own weight while it can still be added to. If the surface looks dry, dusty, stiff, and pale, the slip may not be enough to hold your attachment. Instead the clay may be too soft to hold up to the handling, or you may need fresher clay to join to. You may have to work more slowly and add smaller sections of clay.
A nice-looking seam does not necessarily mean that the surface has been hidden with a lot of blending. Blending the clay too much can distort the area around the form or the attachment, or even worse, you cannot see the problem until later. Once the joined parts are pressed together, just blend them enough. Look for the edge that is sealed closed, no lifted or uneven seam, and that it stays where you have set it. If slip oozes out of the join, remove that extra with a damp sponge or a small tool and not water soaking into the area.
When you are ready to leave it for drying, examine the joined clay from every side as you turn it in your hands. A good attachment should be an integral part of the form, not just added to it. Check that the attachment such as a handle, foot, coil, or decoration is not shifting when you gently move it. Taking the time to make sure the join is good before you set it down to dry saves time later and teaches your hands what good clay joints can feel like.