The basic step to all slab work, is to first work the clay to get a smooth consistency with
no lumps or air bubbles. Then on wax paper, roll out a large pizza-sized section with a standard
rolling pin to the thickness you want. For large slab ashtrays, I suggest about 3/8" thickness.
Then with a knife or needle cutting tool, cut the shape you desire for your ashtray. Now you
have the slab.
The next step is the fun part. You wad up several pieces of newspaper to about the size of eggs, and put them together on a work surface (probably smooth plywood or pressboard). Now take the slab and lay it over the newspaper with the "egg(s)" in the center and let the edges of the slab fall down to the work surface. This is going to be the inside of the ashtray where the paper eggs are. At this time, you can decide if you want larger or smaller eggs and work the edges of the clay down around the center eggs. This is the way you are going to let your slab get "leather dry." Leather dry is the point where it will hold it's shape, but it is still wet enough that you can easily trim, smooth and work the clay. But you are not finished yet. If you want a pattern in the clay on the inside of the ashtray, you would have had to already pressed it into the clay before you lay it on the paper eggs. If you want to decorate the underside of the clay with a pattern, do that now.
The next step is the "foot" of the ashtray. This is what makes it sit level and have a firm position on a table. Roll out a long "snake" of clay, one that is long enough to go around the top of the mound of clay formed by the paper eggs. Apply it to the clay and work the inside and outside of this coil into the slab base. This is important so it won't fall off during the firing. Really mix the coil with the slab base good. I use a butter knife to work the clay together. After doing this, I place a flat surface on top of the coil, and make sure it is parallel to the table surface, and press. This will flatten the coil slightly on the bottom so it will sit level when dry and turned over.
Well, that's it. Let it dry to leather hard, then smooth the edges of the clay as much as you want. You can even scrape the bottom of the coil to even it out and I have also set the coil bottom on a medium grit sand paper and carefully pulled the entire ashtray over the sandpaper. This will further flatten the coil foot and make it exactly level with a flat surface. Now let the clay dry hard for 4-7 days before bisque firing it. After that, you glaze it.
Some people have had luck with glazing raw clay and firing only once. This will work using a low temp clay (like white clay) and glaze, but if the clay comes apart, you have wasted a lot of expensive glaze. Plus, you can still sand even bisque fired clay to make it smooth if you need to, before applying and firing the glaze. I always recommend two steps, but if you are in a hurry and willing to risk it, you can certainly get by with a one step firing and glazing. (You may want to see the complete page on Firing Cones and Temperatures for reference.)
For all other kinds of slab work, just cut the sections of slab that you want and build as if you were using wood and glue. Always work the clay together at the edges to insure a good seal with the clay. Boxes with tops are quite easy to make and make a good gift. I have made many using white clay and a clear glaze. Afterwards, put some decorative decals on the white sides and top and fire into the clay and you can make some very nice pieces. Later on the pottery pages I will put on some illustrations of this method with decals. You can use letters and put a person's name or initials on the piece as well for a permanant and personal momento.