Saturday, November 03, 2007

Types of Clay Pipe

When you purchase your first clay pipe, it is important to know that there are two distinct types of clay pipe: slip cast and press cast.

Slip cast pipes are made by pouring a fairly fluid mixture of clay and water (called slip) into an absorbent mould (usually plaster). A certain amount of the clay clings to the inside of the mould as water is absorbed by the mould; the excess is poured out. The mould is then dried for a few hours or a few days before it is cracked open, at which point the slightly damp clay pipe is trimmed, dried further, then fired before use.

Press cast pipes are made from solid clay. First, a rod is rolled out from a ball of clay, leaving a small bulb with a rod extending from it. A wire or similar object is carefully driven down the length of the rod until just before reaching the bulb. The entire thing is then placed in the mould, and a shaping device is pressed into the bowl part of the mould to force the bulb into the correct shape. The wire is then pushed all the way through into the bowl, the pipe is dried for an hour or two, removed from the mould, dried further, then fired before use.

In general, the clay pipes you see most often in catalogues and pipe shops are the slip cast variety, as these are the easiest to make and also the cheapest, so they represent a very small risk to the vendor - remember that most people don't smoke clay these days, so vendors usually carry them mostly for novelty value.

When you purchase a clay pipe, you should inspect it for the following qualities:

  • seam lines
  • pits
  • airway (esp the entry into the bowl)
  • chips or cracks

Slip cast pipes will usually have marked seam lines. These are the weakest part of the pipe, and sometimes begin to separate during firing or as the pipe begins to age. Press cast pipes often have no seam lines, or seams that are very difficult to make out. In the case of press cast pipes, the seams do not indicate any weakness, but do indicate that the maker wasn't very consciencious with regard to the finishing touches, and the price should reflect this.

Pits in the surface of the pipe indicate that it was press cast, and can indicate weak points in the clay. Clays that have been mishandled during shipping may also have developed chips or cracks. During the heat stress of smoking or cleaning, these are the points that are most likely to give way. A pipe with these flaws is still smokable, but you may want to buy a different pipe if the chips etc are in vital spots such as large flaws on the bowl or cracks/chips at the point where the bowl meets the stem. Chips at the end of the bit and around the rim of the bowl are just an aesthetic problem, however, and you needn't worry about them provided you are paying a very good price for the pipe.

The airway of a clay pipe is a sticky problem. Press cast clay will shrink approximately 20% during the drying and firing process, while slip cast clay will sometimes shrink as much as 35%. This makes it very difficult for the manufacturer to judge the size of the airway before the pipe is complete. Many clay pipes (especially slip cast pipes) have their airway restricted to the point of being unable to pass a pipecleaner through them. As clay pipes don't often need that sort of thing while smoking, it shouldn't pose a problem for you, and shouldn't prevent you from buying the pipe. If, however, you tend to prefer blends that typically smoke wet (heavy aromatics and such) you may want to do the pipecleaner test on any clays before buying them. Remember however that clay is very absorbent (especially slip cast) and that not only the bowl but the whole stem will work to absorb moisture while you smoke. The main issue will be cleaning, which I will cover in a moment. You also need to be sure that there are no obstructions in the airway that will restrict draw unreasonably.

It's unlikely you will get a complete obstruction, but slip-cast pipes sometimes have an uneven interior, depending on the process used by the manufacturer. The easiest way to test the airway of a clay pipe is to run a length of florists' wire down the stem - if it meets any resistance at all, you have an obstruction. Usually, the obstruction will be small and easily removed by gentle pressure from the wire. If it doesn't move with gentle pressure, you will need to gauge where in the stem the obstruction lies, and if it is at least 5 or 6 cm from the bowl you should be able to shorten the stem so that the portion including the obstruction is removed.

In addition to the main airway, however, you need to pay special attention to its entry into the bowl. Sometimes this entry is partially obstructed, and this can usually be cleared easily by picking at it with a pipe tool or some other hard, narrow object (needles and nails work well). If it is a press cast pipe, the wire used to make the airway sometimes fails to make it all the way through to the bowl, in which case there is really nothing to be done.

All of the above flaws can be problematic, but remember: they *are* flaws, and if you didn't notice them before purchase it's almost certain your vendor or the manufacturer will replace it with no problem. It's also useful to remember that briar has flaws too. These are different flaws from the ones usually encountered in briar, but there aren't any more of them, really.

In addition to the basic slip cast vs press cast distinction, it should be noted that some clay pipes have been glazed. A glazed pipe will have the characteristic glassy surface of your fine china. A glaze pipe can be very attractive, since it is a good way to permit more intricate and durable decoration. However, it should be noted that there are some difficulties with glazed pipes:

if the inside of the bowl is glazed, the pipe may not be smokable. The airway may be completely blocked by glaze, which would require a needle file to remove. Other than this, it is likely that a pipe that has been completely glazed, inside and out, will not smoke like a clay pipe at all, but more like a glass pipe. This is because the interior glaze prevents the clay from absorbing moisture and tars as you smoke. A glazed pipe will require more thorough cleaning of the stem as well.

even if the inside of the bowl is not glazed, glazed ceramics of this sort are often made from porcelain clay which is much denser and less porous than the usual sort of clay used for pipe making. This will cause problems similar to those found in a pipe with the interior glazed, and means that cleaning will probably not be able to return the pipe to nearly new appearance.

glazes are generally fired at a much lower temperature than the clay was originally fired. If the temperature was low enough, fire cleaning may cause the glaze to sag, run, crackle or discolour. If the vendor doesn't know for certain, you can hazard a guess: porcelain clays (very fine grained, white clays that produce a dense, light ceramic when fired) and stoneware clays are fired at very high temperatures, and the glazes used with them are similarly hardy at high temperatures. Standard kiln cleaning might not bother them (though of course you will still be taking a risk) and other forms of fire cleaning probably won't. If the clay appears quite porous (like a flower pot) then it was probably fired at a lower temperature, meaning that the glazes were fired at a lower temperature still. Such a pipe will survive fire cleaning, but the glaze will probably be ruined.

On the whole, due to the uncertainties a glazed pipe may well be best left as a display piece. However, I have personally made clay pipes *and* glazed them, and they have worked well. The issue is not whether or not glazed pipes work for smoking, but whether the manufacturer really had smoking in mind when the pipe was made. A pipe that is clearly intended as a display piece will probably not smoke well, and may suffer when you attempt to do any comprehensive cleaning. A pipe that is very decorative, but appears to have been designed for actual use will probably smoke as well as any other clay pipe and will not pose any problems when cleaning it. As usual, caveat emptor.

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1 Comments:

Anonymous Bill Miller said...

Thank You for a wonderful post! this is great information and delivered in a clear concise manner. I have just purchased my first Clay Pipe and was wondering how to go about caring and smoking it. Your post provided excellent information

January 16, 2009 7:09 AM  

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