|What points determine a pipes overall quality?||back|
There are several main points to look for when determining the quality of glass:
1. No cracks! When buying a piece of glass, check it carefully for cracks. Hold it up to the light, and rotate slowly while meticulously examining for cracks. Any pipe that has a crack you can consider virtually worthless, as it will almost surely break there at some point in the not-so-distant future.
2. Thickness consistency. While thicker glass is stronger, and generally better, it is of more importance that glass be evenly thick (NOT overly thick on one side, and thin on the other). To understand why this is so important, you have to understand a little bit about how glass stresses as it cools. You can imagine that glass, like other materials, expands when heated, and contracts when cooled. When two areas of the same piece of glass have different thicknesses, a stress occurs between the areas, as they cool with different contraction rates. (This is because the thicker part retains more heat, thus contracting at a slower rate) This stress diminishes the structural strength of the glass causing it to crack and break more easily upon impact. This stress can be seen using a device called a polariscope. Firing a piece at 1040 degrees, and allowing it to cool slowly and evenly in the kiln (known as 'annealing'), helps eliminate this stress.
3. Structurally sound shape. Beware if your pipe is stretched out, blown out too thin, or narrow in any one spot. Can you visualize where the pipe would break if it fell on a hard surface (obvious weak spots)? As a general rule, convex shapes are sounder than concave shapes. Round is strong. You can take a perfectly round (Pyrex) glass marble, and repeatedly (and forcefully) bounce it off the concrete with no detectable damage. This is because the round shape of the marble is so structurally sound. If you took that marble, heated and stretched it out to a thin rod, you would find that it no longer bounces, but instead breaks. This is an exaggerated example, but it makes an important point: shape matters. Does the pipe you're viewing look like it has an overall stout shape, like it could withstand some impact, or does it have a more fragile appearance?
4. Flow. All the glass should 'flow' together, that is, you shouldn't be able to detect any seams or texture on the pipe by running your fingers across it, it should feel perfectly smooth everywhere.
5. Seals or 'welds'. If your pipe or bubbler is a 'two piece' (meaning that two pieces have been fused together), examine where the pieces come together (called a 'seal'). Seals are one of the more difficult operations facing a glassblower, with much opportunity for error. Look closely at the seal, is it smooth? Do you notice any thickness inconsistency or 'ripples' on the inside of the seal? These are tell-tale signs of a poorly done seal. If the seal is done well, it will not be easy to see where the two pieces actually come together, and you will have to pay close attention to where the color pattern breaks in order to see it.
6. Marbles, decorations. Any marbles or other added glass decorations need to be fully melted in with the rest of the pipe. Look at the seams where the marbles connect to the pipe. Are they consistent looking, or are there 'breaks' in the continuity of the seam? The seams themselves should not be pronounced, but instead should flow 'seamlessly' into the rest of the glass. The more of a seam there is between the marble and the pipe, the more likely it is to break off or cause a crack. Also, in general, a marbles widest point should be where it connects to the pipe. Marbles that 'get wider as they go' (like an upside-down pyramid or cone) tend to break off easily.
7. Aesthetics. Does the placement of the carb allow you to hold it comfortably in your hand? Does it stand up stable without spilling a loaded bowl? Is the carb large and placed well enough to allow the smoke to clear efficiently? How big is the bowl hole? It should ideally be between 1.5 and 2.5 mm. Anything smaller clogs too frequently, anything larger pulls the chunks into the pipe before full combustion. If it's a bubbler, do you end up with stinky-water on your carb finger?
These are just general guidelines that I recommend. If you see an artful piece that seems to violate these principles, don't necessarily knock it. For instance, marbles violate the principle of thickness consistency, as the glass gets thicker where the marbles are, yet there are many marbles that are worth having on a pipe because of the beauty they add. Almost all rules have an appropriate time to be broken, so keep and open mind, using common sense and your best judgment (all things considered) when making decisions.