Sprues Defined:
In casting, sprues are wax wires used to connect the wax model with the casting mold's opening. During burnout, the wax is eliminated to leave channels (called gates) through which molten metal flows into the mold cavity. When spruing a model for casting there are several important points to remember:
  1. The orientation of the model.
  2. The placement, thickness and length of the sprues.
  3. The creation of fillets at the points of attachment.
  4. The creation of reservoirs.
1. The orientation of the model.
In all throwing techniques the metal is forced to move away from its source. This means that it can move most easily forward and laterally, but can hardly move backward at all. So, good spruing involves planning so that all parts of the model are adequately fed through sprues that work with the natural direction of the metal's flow. In other words, since the metal is being forced to move away from its source, the model has to be oriented so that everything is downstream and avoids backflow. It is also important to remember that when poured, investment is a thick heavy liquid and usually contains many bubbles. The model has to be oriented to allow those bubbles to escape. Bubbles in the investment will later fill with metal, creating tiny spheres of metal attached to the surface of your casting.
2. The placement, thickness and length of sprues.
After determining the orientation of the piece relative to the source of the metal, a sprue can be attached to the thickest part of the model closest to the source. This sprue will be the most direct source for the metal, so keep it short and thick. This way, the metal will fill the mold quickly and stay as hot as possible. The reason the thickest part of the model is attached most directly to the source of the metal, is that this will insure that the thinnest (fastest-cooling) parts will be fed by thicker (slower-cooling) parts, which are, in turn, fed by the button. (see the section on reservoirs) In addition to the main sprue mentioned earlier, feeder sprues should be run from the main sprue to any parts of the model in danger of not receiving enough hot metal. This might include especially thin parts of the model, or thick parts separated from the main sprue by a constriction, or parts of the model at a right angle to the main sprue. Since the metal can't flow back toward it's source, feeder sprues will need to be run from the main sprue to any parts of the model that project back toward the source. In general, to insure enough hot metal enters the mold: a) be generous, using as many sprues as you think are needed; b) use as thick a sprue as possible, as thin sprues may cool too quickly; c) use as short a sprue as possible, as long sprues will also tend to cool too quickly. The number, size, and placement of sprues should be balanced against how you will remove the sprues later. Avoid making sprues so short they are hard to cut off later. Avoid attaching sprues to textured or sculpted areas of the model, as it is hard to reproduce textures and shapes in metal that were first made in wax. Avoid attaching sprues to places that will be difficult to reach with a saw or snips. Avoid attaching sprues to the main view of the model.
3. The creation of fillets at the points of attachment.
The joint where the sprue is attached to the model, should be broadened to form a fillet. The fillet does three things. 1) Fillets allows the metal to enter the mold without changing its speed. A constriction will cause the metal to splatter and cool as it enters the mold. 2) Fillets reduce erosion of the investment. Since the investment is very brittle, and the metal enters the mold under pressure, the liquid metal will tend to erode thin flanges of plaster at constricted openings. These eroded bits of investment become inclusions in your cast piece. Fillets round the openings in your mold and eliminate this erosion. 3) Fillets increase the contact area between the sprue and your model, which helps them stay on during investment.
4. The creation of reservoirs

A main consideration in spruing is how metal re-crystallizes as it cools. As metal cools, its crystal structure changes from random, active and expanded, to organized, fixed and contracted. This happens in colonies or grains of metal crystals and leaves voids or pits between the grains. If there is enough molten metal introduced into the system, these pits are filled through osmosis by metal that is still fluid. If not, these pits show up in the surface of the finished casting. Obviously, the last parts of the casting to cool will have no liquid metal to fill the pits. The goal of good spruing is to arrange for the slowest-cooling part of the casting to be outside of the area defined by the model. The easiest way to arrange this is to provide a reservoir. The simplest and most common reservoir is the button. The button is an extra weight of metal included in the charge. This metal remains outside the casting in the cup of the mold formed by the sprue base. As stated earlier, the sprues must be arranged so that thicker (and slower cooling) parts of the casting are feeding the thinner (and quicker cooling) parts. The button becomes the thickest and slowest-cooling part of the casting and feeds all the rest. It is later cut off the casting along with the sprues. The model should almost never be attached directly to the button. Having at least a short sprue between the model and the button will make it easier to separate the casting from the button and insure that the pitted button-metal will not form a part of your finished piece. The exception would be long, thin, wiry models that sprues would only add unnecessary length to. If it is impossible to attach the main sprue to the thickest part of the model, additional reservoirs can be added on the sprues themselves. These on-sprue reservoirs are spheres of wax added to the middle of the sprues that lead to thick parts of the model. These spheres will burn out as hollow spaces that will provide the same feed of hot metal and protection from pits as the button.