by Gerald Clarke
Working Table of Contents
Introduction: Why I began casting at home and why I'm writing this book.
Chapter 1: An Overview of the Cire Purdue (Lost Wax) Process.
Chapter 2: Getting Started; creating the wax model.
Chapter 3: What's a vent? A sprue? Preparing the wax model for casting.
Chapter 4: The Investment Mold.
Chapter 5: Burnout; not you silly, the mold!
Chapter 6: Building the Furnace.
Chapter 7: Metal; some decisions to be made.
Chapter 8: Metal, Furnace and Fear; time to get serious.
Chapter 9: The Pour; an addictive activity.
Chapter 10: Bustout; I think I see something?
Chapter 11: The Finishing Touches- patina, wax and mount.
Chapter 12: Advanced Techniques. Appendix A: The Burnout Kiln
Appendix B: The Furnace
Appendix C: Common Casting Faults
Appendix D: Suppliers
Appendix E: Suggested Reading
Excerpt: Chapter 4: The Investment Mold.
I've often been asked by my students if the mixing process could be facilitated by using a drill with a mixing attachment. I think this would be all right in the initial stages of the mixing process, but I prefer using my hand to get a "feel" of the mixture. Once you've achieved the correct consistency, you must immediately pour the mixture into the prepared flask.
When pouring the investment mixture into the flask, make sure that the mixture is not coming down right on your wax model. The mixture is quite heavy and could possibly break the wax model and ruin your casting. Pour the mixture against the wall of the flask so that the mold mixture surrounds the wax model as it fills. If you've got someone helping you, obtain a long, narrow funnel from an auto parts store and cut the smallest part of the funnel off. This makes a great funnel for pouring the investment mixture around the flask and wax model. The mixture should completely cover the wax model and up to the mark you made earlier on the wire cylinder with the modeling clay. Once, I marked the inside of the flask with a marker and covered the mark I made with a splash of investment when I started to pour. I had no idea where the mark was and had to guess. After bustout, I saw that the base of my mold was about 6 inches thick! No harm done, but lots of extra weight that I didn't have to lift in and out of the burnout kiln.
Once you've filled the flask, you should immediately wash your hands and bucket in the bucket of water you prepared earlier. Again, NEVER wash plaster into a sink that doesn't have a special trap to keep materials from settling in the drain pipe! It's worth noting here that plaster is somewhat caustic and repeated contact with your skin can cause irritation. A long glove or the new invisible glove lotion would help protect your hands if you have sensitive skin. A bottle of moisturizing lotion for use in the studio would also be a good idea.
As the investment begins to harden, monitor how long it takes to set up or harden. If your investment hardened within 10-20 minutes, then your investment mixture was about right. If the setting time is more that 30 minutes, then your mixture was a bit on the lean side. Longer setting times mean weakened strength of the mold. This doesn't mean that the mold is sure to fail, but should be corrected the next time. Finally, it's a good idea to mark the bottom of the mold(s) before they're completely hard. After you get two or three molds in the burnout kiln and cook them for a number of hours, it's easy to forget which is which. Initials, numbers or symbols work to identify the molds. This becomes important when it's time to melt the metal. An idea of the amount of metal each mold will require makes for more efficient casting and less waste of time or energy. In the workshops I've taught, I've set as many as ten molds, all similar in size and shape in the burnout kiln. If you've got several people's molds firing at once, it's a good idea to keep a written record of mold markings and their location in the kiln. Especially since they'll be coming out of the burnout kiln HOT which isn't a good time for confusion or second guessing.
Remove the tin flashing from the mold by cutting and removing the duct tape. Carefully break the mold loose from the plywood and remove the styrofoam cup from the pouring basin. You should now be able to see the end of your sprue and tips of your vents. Once inverted and heated in the burnout kiln, the wax will run out of the mold from these openings.
Excerpt: Chapter 5: Burnout, not you silly, the mold!
The Burnout Kiln
First, let's examine the types of kilns available for use in the burnout process. The key factor here is that a temperature of 1000 degrees F. must be obtained and maintained for the duration of the firing process which can last anywhere from 16 to 72 hours or longer. When compared to the ceramic arts, the burnout temperatures are low and easily reached. Many times, my key concern has been keeping the temperature below 1000 degrees!
In general, you've got two choices here: gas or electric. I've used both and will summarize each.
The first burnout's I did with an electric kiln were at the community college in northeast Texas where I taught sculpture. At that time, we had no gas fired kilns and several electric ones. One of the older electric kilns was not used much because it would no longer reach the temperatures necessary for ceramic firing which were in excess of 1800 degrees F. But, for my purpose it worked great. The kiln was square and its' inner chamber measured 2'x2'x2'. In a kiln of this size, I could burnout a maximum of five small molds at a time. The main drawback to using an electric kiln for burnout is that the carbon released from the burning of the wax corrodes the kiln elements and lessens their life span. That said, an old kiln that is not being used really has no life span, so you're not out anything. I did numerous burnout's in that kiln and it worked fine for that purpose for several years.
One thing that I did to lengthen the life of that particular kiln was to drill holes in the bottom of the kiln and place metal funnels in the holes. Each time I did a burnout, I would place the molds directly over the funnels and capture some of the wax in a pan of water underneath the kiln. This minimized the amount of smoke and carbon produced and thus kept damage to the kiln elements to a minimum. Contrary to what you might think, I did not reclaim all of the wax during these burnout's. At best, I'd say approximately 30% of the wax actually made it to the pan of water below.
By far the most enjoyable experience I had building a burnout kiln was in Taos, New Mexico. I was teaching a bronze casting workshop and found that there were no kilns available for our use. The workshop was being taught at the studio of Wilson Crawford. His house was built with true adobe construction and he had numerous adobe bricks around his studio. Inside of 30 minutes, we had built a burnout kiln with a firing chamber 4 ft long, 3 ft wide and 3 ft high. Because the bricks were only sun dried, the bricks soaked up a lot of heat and we had to ◊add a second burner. Other than that, it was possibly the best burnout kiln, I had ever used. Once heated, the kiln held it's temperature and cooled very gradually. We fired 8 molds of varying size and all burned out perfectly. This experience is a perfect example of the low tech and ancient wisdom that allows this art to be practiced by anyone willing to learn and apply it.