In the world of casting, there are two main ways of making a mold. The simplest is to take an object, shove it into something which leaves an impression, then pour the metal into the impression that was left behind.
This method is often called sand casting, since sand is used as the something that the object is shoved into. Sand is a wonderful casting medium because it is reusable! After you finish your casting, you can pack the same sand around something else to create an entirely new object.
The problem with sand casting is your object needs to have "path of draw" which simply means that there can be no undercuts in your object to allow the object to be drawn out of the the sand without disturbing your mold.
Imagine you wanted to cast a cone. If you hold the base and shove the tip into the sand, you will be able to pull the cone out of the sand and leave a perfect impression of the cone in the sand. This impression will hold molten metal perfectly!
If you held the tip and buried the base, you would not be able to pull the cone out without disturbing the sand and destroying your impression.
Ensuring that you always have a path of draw is important, as it allows the object to be reused should the casting fail and need to be repeated. This lets you make one blank that can be used to create an infinite amount of molds over and over again!
The truth about the sand though, the object is not pressed into the sand, but instead the sand is poured and packed around the object. Then the entire sand box is flipped and the object is removed. It is just easier to visualize shoving a cone into sand then to try to describe packing sand around the cone and pulling the cone from the sand.
The other method for making molds is the lost wax technique. Lost wax is very forgiving when it comes to shape. There can be undercuts, as well as intricate passages between the metal, as the entire item is created in wax! Anything wax can do, the mold can also do, and hopefully, so will the metal!
When you see a bronze statue, where the intricate body could not have been pulled from a sandbox to leave behind an impression, this is the technique that they used. The entire statue is first made in wax and then covered in plaster. The plaster will adapt perfectly to the wax and take on the shape down to the last detail. Once the wax is entirely covered with stone, it is ready for the second step: the loss of the wax!
The mold is heated, causing all the wax to melt out and be lost; hence the name of the technique. Now you have a stone cast of the item you wish to make in metal with a void where the wax used to be. The metal is then poured into the hollow space and allowed to fill all the voids.
Unlike the sand casting technique, where the blank is saved and reusable if you mess up the casting, with a lost wax technique, you only get one try. If you fail during the casting phase, you need to start from scratch and do it all again!
Naturally, you want to make sure everything is working perfectly before you risk destroying a masterpiece of a wax form that may never be fully realized in its metal form! Here is where practice and experience come into play.
If the wax is fully covered in stone, how will the wax escape and the metal enter? Experience will help the craftsman place the sprue in the right places to allow the wax to exit, the metal to enter, and the air to exit the mold. If any of these steps are not successful, the entire casting may be ruined!
Sprue placement is important, and intricate pieces will require intricate sprue designs. In general, larger items will need multiple sprues to allow the metal to enter and flow through the mold. Sprues also need to be placed at the top of the mold, that way they don't spill out metal as the mold is filled.
If you have a long flat item to cast, placing two sprues, one at each end will be a good idea. When you pour the molten metal into one sprue, you will know that it has passed to the other side of the mold if you see it exit the other sprue. This will remove doubt about whether or not the metal made it all the way down the passages.
The lost wax technique can be rewarding as well as painful. If everything comes out well, your casting will be as intricate as your wax model. If anything goes wrong, you will be left with nothing more than a disfigured dream of a casting. Your wax is gone, your mold broken, as your heart saddened as you begin the entire process over again.