In this process slip, a liquid mixture of clay and water, is used to help weld the pieces together. (Sometimes, instead of slip, only water (or spit) is used, or some special formula (see below). Until you experiment and know what works for you, slipping and scoring is pretty fail-safe if done correctly.)

• Lay out & trace or mark where the pieces are to be joined to each other.

• Using any scoring tool of your choice such as serrated scraper, comb, rake scoring tool, fork, needle tool, knife, or other sharp tool, score grooves into the clay. Cross-hatch the grooves by scratching the surface to make a rough area.

• Dab or spread slip over the scored areas using, a brush, a knife, your finger, etc. Make sure to fill in all the grooves so you create no air pockets & take care not to "rub out" your score marks. (NOTE: Some potters reverse this process, putting slip on first then scoring through it, thereby cutting the slip into the clay as they do.)

• If the clay is on the drier side of leather hard, score and slip the areas again to really work in the slip and make the pieces sticky.

• After both pieces are slipped and scored, you might wish to let them sit a moment or two until the wet sheen of the slip dulls down. This will make them more sticky and less slippery when you push things together.

• When you're ready, firmly press the pieces together. If possible, gently press down on one piece above the joint or wiggle it a tiny bit as you push it on to help seat it and get rid of any air pockets.

• Welding - This step is generally only necessary or suggested if you need structural support or have a "thin" connection where you are joining pieces. To create a "weld" you will roll a relatively thin coil, or cut a thin strip of clay, then slip & score it on the seam of where you attached your two pieces. Then you will use a tool such as a wooden modeling tool, popsicle stick, etc. to blend the coil into &  across the seam to strengthen the joint. 

• Finally you will smooth or blend the two pieces by wiping away the excess slip and either using a brush, your finger or another tool such as a wooden modeling, or bone tool to gently blend across the two attached pieces. You will always want to wipe away the excess slip or slurry in the smoothing step, but you may not always need to blend together adjoining pieces.  

Resources: Edited from Mrs. Dowling original text 

1.) More Moisture = Better - Wet the clay is, the easier it is to join.

Why... The more water between the clay particles, the more they can slide around and intermingle when clay pieces are next to each other, and the more soft and plastic the clay is, making it easier to physically blend clay pieces into one another.

2.) Join Like to Like: The pieces being joined should be the same dampness.

Why... When joining clay parts together, they should be equally wet (have the same moisture content). As clay dries it shrinks - physical water between the clay particles evaporates, and the clay particles draw nearer to each other. If a wet clay piece is joined to a drier clay piece, the wet clay piece has more shrinking left to do than the drier piece, and so (as it does shrink more) it will create stress and want to crack away from the drier piece. Unknowing beginners may be able to join a wet piece of clay to a near bone-dry piece (typically something they have been working on for days and have let dry out too much), but in the end, no matter what they do, the wet clay will shrink more than the drier clay and all their effort will usually go for naught. The simple solution is to spend a little effort before attaching things to re-wet the drier piece, and/or dry out the wetter piece, until the parts are evenly wet, then join them. Paying attention to this rule will make working with clay 100% less frustrating.

Resources: Sam Scott & Mrs. Dowling ​