Tapping dice are one very special kind of loaded dice. They are called "tapping" dice because the gaff can be activated, or de-activated, by tapping the dice onto a firm surface. The gaff consists of a small weight, inside the cube, that can be shifted from side to side, with the right momentum.
Due to the fact that these gaffed dice contain a miniature mechanical apparatus, inside of a secret hollow part of the cube, these dice have almost mythical status - and are highly overrated. Part of the psychological appeal lies in the fact that the miniature mechanism is completely hidden from view and as we all know, curiosity does miracles in the marketing department.
Thousands of these dice were sold throughout the booming era of crooked gambling suppliers, but unfortunately very few survive. One of the reasons why I think so few survived is because from the outside these dice look just like any other dice. In other words, when the original owner of the dice is not longer amongst the living, the heirs are very unlikely to ever figure out the dice that "Uncle John" had were crooked. But if the heirs knew these innocent looking cubes had a secret hidden mechanism, they may have to fight very hard to overcome the urge to smash those creatures to pieces, to see what is inside.
Fortunately, if you ever come across any kind of tapping dice, you will not have to smash them to pieces, just to see what lays inside, because you have this web page to put your curiosity to rest. No more sleepless nights thinking and trying to figure out what is inside those damn tapping dice. Just look at the pictures and you'll know.
The kind of tapping dice that use a mechanical weight can be put in three groups: one-way dice, two-way dice, and three-way dice.
The dissected die in the photo above is a one-way cube. This means that the weight can shift to one side of the cube and therefore favor to land on that side. A two-way cube would have the axis from edge to edge and therefore, when the weight is shifted into one edge, favor to land on either one of the two sides that share the weighted edge. And finally, a three-way cube would have the axis from corner to corner, thus making one of the corners heavier and favoring to land on either one of the three sides that share the weighted corner.
Next photo shows some of the stages in the manufacturing process of tapping dice. The first cube is a metal tool around which the hollow cube is built. The cube is assembled by gluing together six separate sides and shaving off the excess material at the edges, as the cube is being put together. In the final two stages you can see a fully assembled, and loaded, cube that does not have any spots, yet. At this stage the cube is slightly oversized. All the remains to be done is to mill-out the spots, as seen in the last cube, fill the dots, once side at the time and shave off the sides down to standard size.
Below is a photo showing several tapping dice in various stages of decomposition, as well as a completely exploded view of a two-way cube, that shows the internal mechanism.
Some other kinds of tapping dice use mercury, but in all honesty I have never seen those cubes, so I cannot offer any insights. Some of these gaffs have an almost mythical status and often it is hard to separate facts form myths. One could say that the mercury cubes are almost like Yeti, the famous Abominable Snowman that (supposedly) lives in the Himalayan region of Nepal and Tibet. Everyone talks about this mysterious apelike creature, but no one ever gets to see him. For all I know, mercury cubes may just be the same kind of "creatures" as the Yeti, the Holly Grail, flying saucers and many other myths that have boggled the minds of those looking for definite proof of existence.
Finally, there is another kind of tapping dice that are filled with granules of a heavy metal (lead, tungsten, gold, platinum...) submerged inside of a highly-viscous oil. The dice don't really have to be tapped. To set the dice one is required to simply turn them to the desired side and wait a few seconds for the granules to shift, inside of the hollow chamber. The highly-viscous oil makes this process slow, so that the granules don't bounce around as the dice tumble. The advantage os such dice is that any side can be set, but the disadvantage is that the dice can never be set to roll fair, as it is impossible to force the metal granules to be distributed evenly throughout the cube.
One of our members (who asked to remain anonymous) was kind enough to submit a photo of his own, showing a fully functional tapping die with a clear side. This cube was obviously made for some kind of demo work.
Below is a clipping from an old HC Evans Blue Book, crooked gambling catalog, showing a listing for tapping dice. Note that the listing says that their dice are "made in white, red or blue opaque celluloid." One of the images above show uncompleted dice made in these exact colors. These are actually uncompleted dice from HC Evans, i.e. the exact dice described in this listing. Some of these dice have spontaneously decomposed over time, despite the fact that the company called them "improved" at the time. But in all fairness to HC Evans, there is really nothing that they did wrong when they were making these dice. One of the biggest problems with tapping dice lays in the fact that the sides are glued together, instead of the cubes being cut from one piece. Over time, some adhesives are known to lose the battle against higher forces of nature. The biggest enemy of tapping dice is thermal expansion. All materials expand and contract as temperature changes. The thin layer of adhesive between the two celluloid pieces will expand and contract at a different rate then celluloid, because all materials have different thermal expansion rates. Over time, thermal expansion will produce hairline fractures between any glued surfaces and as more time goes by these cracks will increase. Thermal expansion is also the reason why all CDs and DVDs will eventually spontaneously crumble. Nothing lasts forever.