Lost Wax Instructors Notes
Foreword
I have taught this a few times, and what follows seems to me the most efficient way to get the subject across. The class is broken up into two sections, and preferably these should be about three hours long and separated by as little time as possible. - Jim Bredt 20231223
What to tell the students
If they have a project they want to work on, tell them to bring it, if only for discussion purposes. For their fist mold though they should stick to something inside a 1" cubic envelope. Advise them that their first casting has a high chance of failing, so they shouldn't put too much work into the pattern.
Tell them to wear closed-toe shoes and natural fiber clothes.
Tell them to bring their materials fee.
Before the class
Make a set of molds and burn them out. I prefer a "Julia Child" method which requires one mold per student, and possibly at least one extra as a back-up. If you burn them out the night before the first class, you can have them hold at the preheat temp (900-1100F) until class. If not, you will need to preheat the molds for 2-3 hours before class time.
The patterns don't matter much, but it's nice to send people home on the first day with something nice. Look through the box of stored rubber molds and use the wax injector to make several patterns.
Check to make sure all of the equipment is operational. Perform a vacuum check on the casting machine: switch it on and put your thumb over the inlet hole. The gauge should go to a full vacuum in 2-5 seconds. If it takes longer you may need to clean out the oil filter inside the machine.
Make sure you have enough PPE for everybody, especially dust masks and plastic gloves.
Setup for the First Class
Set up wax-working tools: soldering pencils, knives, alcohol lamps. Also wax: sprues and "sticky" wax are a must. Turn on the wax injector at least two hours before class.
You will also want a set of clean flasks and rubber bases, one for each student.
Metal: I used to make the first castings from scrap brass, which is abundant as of this writing. You may want to poll the students if they have preferred metals they want to use. Budget 100-150 grams (bronze) per student.
Starting Lecture - One Hour: Safety, Process, and Materials
Probably the students will want to fuss with spruing their patterns during the lecture. Be sure to call out the most important details, especially safety to make sure you get the point across.
Safety
In general the safety protocol is the same as for the welding shop in the Antwerp building.
Eye protection is most important It is good at the very start to put them in the mind of the consequences of a steam explosion. Hand out goggles to everybody and be careful to police them throughout the class.
Dust masks Not just for the silica dust from the investment, warn of heavy metal fume from the melt, especially zinc over molten brass. Also there is a pump oil smog that is emitted by the casting machine. Dust masks should be worn pretty much all of the time when performing tasks in the back of the shop.
Gloves, Aprons Remind them that the gloves aren't insulating
Natural fiber clothes If you spill molten metal onto polyester it turns to napalm.
Closed-toe shoes Metal spills of any significant volume are very rare, but the consequences are severe.
Handling hot things Things don't have to be glowing to be hot enough to really cripple you. Use tongs. Assume everything on a hot-bench is hot unless you know better. Test for temperature with the back of your hand.
If you are preparing to deliver this lecture, then please review the more detailed safety document on the Casting Shop Safety page.
Lost-Wax Process
Patterns Can be wax, 3DP plastic, or vegetable material. Some woody materials leave ash, but frequently there isn't enough to cause problems. Slowest to burn out is starch, which turns to charcoal during firing. The detail of which the process is capable of rendering is about 0.1 mm, or about 0.005 inches. This is the lower limit of what the unaided human eye can perceive.
Spruing Usually a single length of wax rod that joins the pattern vertically to the button on the rubber base. There are a few nuances: Join the sprues to a spot on the pattern where you don't mind filing off and polishing the surface. In vacuum casting metal flies (not flows) more or less in a straight line, so avoid sharp bends wherever possible and avoid making metal flow uphill. Diameter of 3-5 mm is okay for almost everything.
Remember that the sprued pattern is built upside-down. The rubber base forms the pouring cup of the mold which is at the top during pouring. At this stage imagine that the metal enters from the bottom of the structure and flows towards the top.
Some materials are very difficult to bond to a wax sprue, so for them it is prudent to bring a needle and some thread to stitch through the pattern and reinforce the joint with the sprue. Woody patterns are quite buoyant in the investment, and they can break free when the investment rises around them. There is additional buoyancy during the vacuum stage.
Investment A powdered product that is mixed with water and then poured into the flasks to embed the patterns. Draw a picture if you feel like. Castable and resists the heat of molten metal. Mixture of gypsum and silica powder. Crystalline silica is hazardous to breathe, and students need to manipulate the powdered investment under the hood wearing a dust mask. Explain scooping techniques to minimize dust evolution.
After mixing the investment is degassed on the vacuum table under a bell jar. This ensures a minimum of porosity on the surface of the mold cavity. Timing is very important, all steps must be completed before the investment sets up, in about eight minutes. If you find there isn't enough time, the best way to slow down the cure is to mix with cold water.
Burnout Mold is heated gradually to first evaporate moisture, then melt or pyrolyze the pattern. The end result is dehydrated investment with a hollow cavity that coincides with the shape of the original pattern plus the sprue. Temperature is held at 300 F to evaporate, 650 F to pyrolyze stuff like nylon in 3D prints, and then finally at 1350 F to remove all traces of wax residue and organic material. 2-3 hours is acceptable hold time for 3" molds. Hold times increase as L^2 for larger molds.
Preheat For most molds, 1000-1100 F is quite satisfactory. Soak for 2-3 hours. In a pinch you can hold the mold at the maximum 1350 F and pour into that. If you don't preheat you risk losing detail or having the metal freeze before the mold is completely full. Strongly geometry dependent.
Pour Metal Discuss the "Buddy" system for safety. Pouring metal is a two-person operation. It doesn't have to be but you need two people there anyway in the unlikely event that one of them suffers a debilitating accident.
Molds are removed from the preheat oven in sequence and placed on the vacuum table, vacuum is applied to the space at the bottom of the mold and molten metal is poured in.
The action of the vacuum table is to draw air through the pores of the investment mold so that the pressure of the atmosphere forces the metal into the sprue and jams it against the surface of the mold cavity. This way the metal is forced into the finest details of the mold. The metal enters the mold in a tiny fraction of a second and solidifies in the cavity over several seconds that follow. The "button" at the top of the mold is almost always the last part to solidify.
Metal should be melted as quickly as possible and poured as soon as it is fully melted. You can hold a melt at temperature for up to an hour without much harm, but the quality will degrade.
De-invest Wait until the metal has had a chance to cool in the mold for a little while, but the mold is still hot. Grasp the mold with tongs and FULLY immerse in a bucket of cool water to disperse the investment. Partial immersion results in steam explosions. Hold under water until it stops making noise. Casting can then be fished out. Remaining investment must be picked off or wire brushed, or removed in an ultrasonic cleaner.
Pattern Materials
Wax, resin These are fully organic and burn out without leaving ash.
Generally these are molded or 3D printed, but you can carve them too. The word "resin" covers a wide variety of materials, and some burn out more slowly than others. Stuff like polypropylene and acrylic burn out very easily. Others like polyurethane or nylon burn much more slowly. Stuff like epoxy is in the middle.
Plants, wood, meat, etc Very often these leave some inorganic residue. Most plants leave a thin wisp of ash that can be blown out of a mold cavity provided the sprue is big enough to allow it. Structural wood leaves a bit more ash. This is a good time to show the chicken foot, which has cavities in the casting left by the bones. For contrast show a pine-cone casting that although it has substantial ash, it rarely leaves any visible defects in the castings.
Metals
Molten metals in general should be thought of as liquids with viscosity similar to water or light cream, but with a surface tension about 20-50 times higher than water. And hot enough to kill.
Precious metals Gold, silver: these work well, and in fact the investment is optimized for these metals. Argentium is a silver alloy with germanium. Stirling is silver with 2.5% copper. Carat gold (below 24 ct) is alloyed with either silver, copper, or both. Platinum can't be cast in the standard investment molds.
Copper and alloys Pure copper is poured at about 2000F, just at the temperature limit of the electric melting furnaces. Alloys all melt at lower temperatures. You want to pour at about 100F (or more) above the normal melting point of the alloy. These data can be found elsewhere. Brass is copper plus zinc, "Ancient" bronze is copper plus tin. Silicon bronze you can guess. Also found are nickel and aluminum alloys. Copper alloys invariably contain lead, very often around 1-2%. Another reason to handle molten metal quickly.
Aluminum melts at a relatively low temperature but it is actually somewhat challenging to cast. Aluminum typically has a higher viscosity and a higher surface tension than other metals described above, so you need to sprue it rather generously. I have never tried aluminum in this process, so I can't say what the preheat temperature ought to be or when it is okay to quench the mold in cold water.
Pewter, "White" metal Alloys incorporating tin, zinc, bismuth, antimony, lead, and often silver. These pose no problems in lost-wax casting. Don't bother preheating the molds. Castings may be difficult to remove from the molds as a result.
Ferrous alloys Incompatible with our gypsum-based investment.
Gypsum Plaster
This is the material that bonds together the investment molds. It is made from the mineral Gypsum, CaSO4-2H2O (dihydrate). Gypsum can be found in monumental crystals in caves, and tourist traps usually bill them as "cave of swords" because of their long columnar shape. Calcined at about 300F to CaSO4-1/2H2O (hemihydrate) or thereabouts. The calcined phase is much more soluble in water than the fully hydrated phase, so when you mix the powdered product with water the hemihydrate dissolves and recrystallizes as dihydrate. The long columnar crystals lock together and form a mechanical network very early in the reaction, which only gets more and more solid as the reaction proceeds.
When the mold is fired in the kiln, first the residual water in the pores is driven off, then the water of hydration at progressively increasing temperatures. By 1000 F all of the water is gone. It is only after that treatment that it is safe to pour molten metal into the mold without risking a steam explosion. If you continue heating, at about 2200 F the gypsum begins to evolve sulfur dioxide as it decomposes to lime. In this reaction, about 30% of the weight of the gypsum is volatile, so pouring high-temperature metals (such as iron or platinum) into a gypsum mold can cause an explosion similar to a steam explosion.
Investment is typically about 1/3 gypsum and 2/3 filler (usually silica) by volume. Because the gypsum is so effective at building a 3D network, there is comparatively little needed to hod the mold together, and the rest of the bulk can be made up of more refractory components.
You want the reaction to proceed at a known, well-controlled rate. For that you need to mix the slurry well, but not too much. Over-beating the slurry will cause it to cure too fast. Likewise the presence of foreign material, especially flakes of old investment, will tend to accelerate the cure and may lead to differential shrinkage and cracking. Warm mix water accelerates the cure, cool water retards it.
Keep oil away from plaster powder. Many oils, especially fatty acids, can prevent it from curing at all. Salts generally tend to accelerate the cure.
Graphite
This is the "wonder material" in the casting shop. It resists heat and molten metal doesn't react with it, so it is the ideal material for making crucibles. There are also gaskets of graphite paper on the casting machine, and graphite rods are used to stir the melt.
Rest of the First Session
Pour Metal Into the Molds
You should have molds preheated and ready to pour at the start of the class. If you or your assistant are clever, they will have started melting metal about halfway through the lecture, approximately when you are talking about molten metal handling. That's a really good cue to remember.
To wake up the students after the lecture, start pouring metal. First do a walk-through of what the metal-handler and the buddy do. Best is to actually pour a mold with an assistant. Then break the group up into pairs and have each of them pour one mold and buddy for another pour. Demold the parts and let them keep their castings.
Remember to emphasize the correct handling of hot molds with the tongs. Molds should be carried sideways in the tongs so they don't slip out. Rub the bottom surface of the flask on a brick before placing it on the hot side of the vacuum table. Press the mold down on the graphite (or silicone) gasket while the pump is running and push down until you see the needle of the vacuum gauge start to climb. It should be well past the halfway point when it stops traveling.
PPEs for this step are: Eye goggles, Dust masks, leather welding gloves, and leather apron. Both the metal handler and the buddy MUST be dressed in these to safely operate.
The metal handler tends the melting furnace and tells the buddy when the heat is ready to pour. This is when they can feel that all of the material in the crucible has liquified.
The buddy turns on the vacuum pump, then takes the mold from the furnace and scrapes the bottom surface of the flask and then they place the mold on the high-temperature side of the vacuum table, as described above. Then the buddy walks to the other side of the metal handler and assists in lifting the crucible from the furnace.
The metal handler is then free to pour the metal into the mold.
The pour should be done relatively quickly without pausing. Sometimes if there is a lot of flux in the crucible, the metal can come out in a surge. You can scoop out excessive flux with a graphite rod.
Finish Spruing and Invest the Students' Patterns
After the excitement of pouring metal, the students need to make the molds for the second cycle, to be poured in the second session of the class.
Everyone needs to have at least one pattern to invest. If any of the patterns are very bulky, weigh them to estimate how much metal they will need. Bronze weight about 8x as much as wax. For small patterns this is less important because the sprue and button will take up about 50 grams of metal anyway.
Take the students one at a time through mixing the investment, degassing, pouring into flasks, and second degas. They shouldn't need you to demonstrate these operations yourself, just walk them through it. Hopefully everyone should have an idea about how to scoop a powder into a bucket until it has the correct weight. Same with stirring.
Remember to wrap the tops of the flasks with tape to make room for the bubbling during the degas step. Their names and the date go on the tape. Warn them that unlabeled molds can be taken and used for random demos without warning.
Carry the molds from the rubber base plate. If you don't it can slip off during handling and dump the contents onto the floor.
If using a cordless mixer, then only mix for about a minute and don't sweat lumps in the mixture. Too much mixing with the power tool will make the investment cure too fast. If you want to eliminate lumps then don't use the mixer, put on gloves and mix by hand, feeling and breaking up lumps with your fingers.
Degas under a bell jar. Press down to ensure the vacuum takes, and keep under vacuum for about 1 minute, or until bubbles start to subside.
Remember to vent the chamber before turning off the vacuum! If you do it in the wrong order, the pump oil foams up and fouls a filter buried inside the machine. You will need to do some surgery to open up the filter housing and clean off the sintered metal filter with alcohol or some other solvent.
Pour the investment down the side of the flask, not over the pattern. You want to have the pattern get covered by a gradually rising surface of investment without a lot of splashing. Don't fret about lumps. There's nothing you can do about them in any case.
Fill the flasks up to 1/8" - 1/4" BELOW the edge of the flask. When you put the molds on the vacuum table you want a hollow space here to distribute the vacuum.
Degas a second time, this time for at least a minute, possibly two. Vent the bell jar and set the flask aside to cure.
This process must be repeated sequentially for every student, and it takes roughly 1/3 - 1/2 of the class time to complete. If you have an assistant, then you can take turns.
Curing of molds takes roughly one hour. When this is complete you can place the molds immediately in the kiln and start heating them, or you can wait for years before completing the process and cast them. Very fresh molds may suffer a little bit in strength, but recall that the first few hours of the kiln cycle pass through a range of relatively low temperatures so the plaster has a chance to fully crystallize before the water starts to evaporate.
DO NOT PLACE MOLDS IN A KILN THAT IS HOT! If you try to start the kiln program when the kiln is above about 200F, it will skip past the early part of the program and not burn out properly.
END OF FIRST SESSION
Before the Second Session
Burn out the class molds from the first session, and arrange to have them up to the preheat temperature by class time. Make sure you have enough of the correct metals, and appropriate crucibles, to pour.
Make sure the molds are organized such that you can have each student pour their own piece. You can have them scribe their names into the investment, but the paper tape labels are also good for this.
Second Session
This is essentially a repeat of the first session, minus the lecture.
Students should be able to perform a whole molding and casting cycle (not necessarily in sequence) wihtout too much additional instruction. Obviously, you need to be flexible here because some students are more adept than others. You can only go as fast as the slowest student in the group.
You should give a lesson on programming the kiln controller. Especially show how to select a program and review it prior to running the program. Since the kiln programs can get changed by other shop users, it is very important to understand how to review the settings and make changes as needed.
This session is when tool-testing might be performed, as desired. Depending on the requirements, the students should all be capable of working SAFELY, if not efficiently, with no prompting.
During this session you may also demonstrate the operation of the wax injector and discuss the use of the vulcanizer for making rubber molds, or use of two-part molding compounds. Remember that the injector takes two hours to get fully up to temperature.
You may also demonstrate the use of the torch to melt metal in one of the ladles, if time permits.
Finally, you can discuss the students particular projects. For the instructor this is the most interesting and rewarding part of the the class.
