So you have a large piece that came out of a mold and it has a hair line crack. These are usually caused by pieces drying unevenly. The piece gets taken out of the mold and it is set on something soft so it will dry enough to stand upright. We use old pillows, but they do not allow air to dry the side it is resting on. Sometimes the piece warps to the dry side or it may crack due to the stress from uneven drying. This can be fixed, but it saves time to prevent this. Pay close attention as the piece sits and rotate to the unexposed side for a while.
To repair these cracks use a small paint brush and wet the area with water then drip and paint slip into the crack. It may take several passes as it dries, but it can be fixed. Use the brush or other tools to get a nice finish and let dry again. Repeat until you're happy.
Another problem you may find is air holes just under the surface of a piece's base. Mixing the slip before you pour to remove air helps eliminate these. If you find some holes they can be repaired the same way as cracks. It is easiest when the clay is wet green to make repairs because it does not crack like it does when it is dry. First: Open up the hole; Second: Put in some wet soupy clay; Finally: Follow up with some soft clay you can work with your fingers. By rubbing over the area with your fingers it helps pack the clay so it doesn't shrink, crack, and settle.
Remember wet green clay is the easiest to work when making repairs or modifying pieces. To see it up close and personal stop in and see or do a workshop. We would love to show you how. Here are some photos of a repair.