Lost Wax Casting Process
Hello, I'm Joe Silvera and along with anot we are teachers and owners of the Silveria Jewellery school in Berkeley California. Today we're going to look at lost wax casting and summarise the basic process.
With lost wax casting you start with a basic model and end up with a precious metal casting. Let's see how. First you make your wax models, these are exact replicas of what you want in the finished metal casting. They get sprued to a base so that they can be surrounded with special high temperature investment which will form this first mould. During the burn out the wax will be melted out and then the void will be filled later with metal.
But first you have to start by sprueing all your models into a little system, kind of like a tree with the trunk coming from the base which is the button and then going onto the branches which are the sprues and at the end of the branches are your models. This is the steel flask that goes over it to hold the investment in place. Let's see the next step. Now we're going to invest our wax models so first we're going to spray in some spray debubbliser in there. This helps to keep air bubbles from sticking to the models which will turn into balls of metal when you cast. We put the flask back on and we get ready to mix the plaster investment.
So this is what is going to hold our models together during the burn out and later keep the void together so that we can fill it with metal. Here I have carefully measured some water and the right amount of investment and I'm mixing it in a rubber bowl. You have only about 9.5 minutes to complete this before the plaster is in the wrong state and almost hardened and so you cannot pour it around your pieces. First we mix it a little bit by hand and then we have to remove all the lumps so that it's a smooth mix, so we're going to do that with a simple egg beater mixer.
Now we vacuum the investment before pouring it so that we get as many of the air bubbles out as possible. So this is called an investment machine. Once this pressure hits about 25 and above then it will start to froth and the air bubbles will start to come up to the surface. Hitting the table and vibrating it will help the air bubbles to come up. Now after a couple of minutes we release the pressure and we can pour the investment over our models. So I have three flasks here ready to go and now I'm going to pour the investment over the. The tape is so that in the next step when I vacuum it again that the plaster doesn't overflow. When you pour, try to pour down the side of the flask so you don't encourage more air bubbles to form on the models like inside of openings or cavities.
Now we have to vacuum the air bubbles again just incase we form some, so we're going to put it back into the vacuum machine. Again vibrating the table once it reaches the correct pressure can help to release the air bubbles. Here we're done after another couple of minutes of vacuuming. As we release the pressure, turn off the timer and now they have to sit and cure for about two hours before we put them into the kiln for burn out.
The flasks will go into the kiln and burn out over about ten and one half hours. They will slowly ramp up to the correct temperature and this will melt out all the wax leaving the void for us to cast our metal into. Well it's about 11 hours later and now we're winding up the centrifuge for casting. So you wind it four times around and then set the pin. This has been carefully balanced so that the weight of the flask and the weight on the other end are balanced so there is no vibration when you throw them out. So now I am melting the metal in this crucible. When the metal becomes moulten and looks like quicksilver then we are going to be read to put in the flask. So this metal looks ready to go and beautiful and now I'm going to get the flask and load it in the centrifuge.
Take it out of the kiln and place it carefully in the cradle holder and then line it up with the crucible. Just making sure that the metal is still moulten and clean and now we are going to release the centrifuge and spin the metal into place. So the centrifuge makes it easy to accurately throw the metal into the mould and the spinning keeps it in place while the metal is moulten. Now that it has spun for a couple of minutes I'm going to slow it down and remove the flask so that we can see the button. The only part you can see right now of the casting, the bit at the end. That is the button on the end of the flask and it's very hot right now. Let's see how hot it is. I'm going to place it in a dark place so you can see the colour and that's how hot the metal is right now. So we don't want to quench the mould while it's this hot. We want it to go down below red heat before we quench it. Now we're going to quench it in water which will remove the plaster and release the metal models from this mould. So this mould, the plaster mould, the investment mould is destroyed every time you do a casting.
Later you can make rubber moulds of your castings to then make reproductions. So first we will remove the flask, clean it off a little bit. Now lets find those models that fell out. Most of the plaster will have dissolved off from the intensity of the heat on the metal and water and there are our metal models.
Time to remove the models from the sprues. Here I am using special pliers that can cut through 10 gauge sprues and now I have some thicker sprues to cut so I'm going to use these sprue cutters. These will cut through some of the thicker sprues like so. Any thicker sprues can be sawn off. Now you have to grind off the little nub of the sprue so I'm using this little grinder made of separation disks to carefully get back to the model and then I want to remove the marks from the grinder. I'm going to use a white silicon polishing wheel to smooth the surface and get rid of the grinder marks so that it's ready for mass finishing. This is what it looks like after polishing with the white silicon wheel.
I use a tumbler loaded with mixed shapes of stainless steel shot to burnish all of my models for easy polishing. So let's take a look at some pieces we're going in another batch here. These are some silver rings and the shot just burnishes the surface and takes it up to a nice shine with no abrasives, so no metal loss. This is a good first step for polishing and if the models are in good shape it's a nice finish, like these. So these are some models after casting and after having been polished. I hope you enjoyed this short presentation and thanks for watching.
Wikipedia has a very comprehensive article on lost wax casting, covering different mediums and the history of the art.