From the January/February 1997 issue of Clay Times
Spouts that drip or don't drip have long been something of a mystery to many potters. The problem is sometimes minor when a spout has one or two drops dribble down the outside of the pot when pouring is stopped. But occasionally some of us will make a pot that dribbles a continuous stream of water down the outside of the pot while simultaneously pouring a stream of water where it is supposed to go. What is the difference? What is going on?
While I have not conducted an exhaustive search for the answer to this question, I have looked through many, many books on the general subject of pottery to learn what is known about this problem. Most authors simply ignore dripping and dribbling and, instead, focus on the mechanical aspects of hand building or throwing spouts. A very few do address the subject, though. For example, Susan Peterson, in her book, The Craft and Art of Clay, writes, "The spout needs to be wide enough at the pouring edge not to gurgle, have a sharp lip to prevent dripping, and be attached so that the open end is higher..."
Michael Casson, in his book, The Craft of the Potter, also addresses the challenge of drip-free spouts: "The rim's edge is very important for this operation and potters get up to all sorts of tricks to achieve a good-looking lip that also pours well...This gives rise to a nice rising lip with a sharp edge that will cut off the liquid..." Again, he focuses on sharp edges.
Unfortunately, having a sharp edge is not the complete answer. I have made more than a few spouts with sharp edges that still drip or dribble. (I suspect most of us have!)
If the spout tip is horizontal or pointing downward when liquid begins to flow,
the spout will not drip.
By far, the best discourse I have found was given by Daniel Rhodes, in his landmark book, Pottery Form. On page 137, he addressed the subject:
"The spout need be only long enough to give a well-directed stream of tea into the cup. Usually, it protrudes no more than about one half of the diameter of the body of the pot. It can either be a tapering or a tube-like form. Excessive taper, that is a very wide base narrowing down to a small opening, may create turbulence in the liquid and cause gurgling. The inside of the spout needs to be at least as large as the diameter of, say, a fountain pen, and it can be bigger than that and still pour well. It should be smooth inside, without constrictions. At the end, the spout will terminate in a sharp lip which cuts off the flow of tea and prevents dripping. The direction of the spout at this point should be roughly parallel to the table; then, when the pot is tilted, the area just under the edge of the lip of the spout will be uphill to the tea. A feature which helps to prevent dribbling is a little ditch or channel cut on the inside of the spout beginning at the very edge or lip and running back a bit into the spout. When the pour is cut off, the tea tends to run back down this little groove instead of down the outside of the spout."
This is very helpful advice. Peterson and Casson also point out, as many of us have heard from one source or another, the importance of a sharp edge. Yet I believe Rhodes' statement about the direction of the spout is often ignored and is what gets most people into trouble. In my opinion, his remarks about direction, however, need a little expanding to assure no-drip, dribble-free spouts.
In my experience, the direction of the end of the spout, when the pot is full, tilted slightly, and just starting to pour, needs to be horizontal or pointing slightly downward. This is illustrated in the drawing above. As you look at this illustration, think about what is going to happen if the spout shown were cut off at the lower arrow instead of being the length and curvature illustrated. It is not hard to visualize that the stream of liquid would split into two parts, with some going where it was intended, and the rest dribbling down the surface of the spout. In the simplest terms, this is just gravity doing its job.
Exactly the same principles (sharp edge and horizontal direction) apply to open spouts such as those on pitchers or gravy boats, although in open spouts, it seems easier to achieve. I will hypothesize that thrown spouts are the biggest dribblers because they are usually straight (pulling or bending a slight curve into them after they are thrown would be a big help). They are often so short they have to be mounted high on the pot in a near-vertical position to get their opening above the top of the liquid in the pot.
One additional point needs to be mentioned. The wettability of your particular glaze or clay surface by liquids can also be important in determining whether or not your pots dribble or drip. Carefully place a drop of water (a medicine dropper is the best way to do this) on a horizontal section of one of your glazed pots. Does it stand up tall like water on a freshly- waxed surface, or does it immediately spread out and run?
Everything else being equal, a glaze or clay surface that wets poorly (the water beads up on it) will be less likely to drip or dribble. With such a surface, you may not have to be as rigorous about having sharp edges and horizontal lips as described above. Conversely, if your glaze surface is very wettable, you may have to have a sharp edge and a significant downward slope to the lip of your spout in order to achieve a drip-free pour.
I know the correct way to make no-drip, dribble- free pots is not widely understood, because I have seen several award-winning teapots in recent issues of various craft magazines that I know do not meet the criteria described above. I would bet some of them are serious drippers and apparently have been judged only on their appearance and not on their functionality. While I have verified that these criteria work experimentally in my own studio, I would welcome comments from other potters on what works or doesn't work for them.
John Hesselberth is a retired chemical engineer with a strong background in fluid mechanics. He now makes pottery full time in his West Chester, PA studio. Comments may be sent to John c/o Clay Times, PO Box 365, Waterford, VA 20197.
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