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.

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 waxworking 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.

Beginning lecture, 1 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

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. 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. 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.

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. 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. 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 work under the hood wearing a

dust mask. Explain scooping techniques to minimize dust evolution. After mixing the investemnt 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 comleted before the investment sets up. 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 300F to evaporate, 650F 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: Discuss the "Buddy" system for safety. Pournig 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 has a debilitating accident. Molds are removed from the preheat oven in sequence and placed on the vacuum

table, vacuum is applied and molten metal is poured in. 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 dispers the investment. Partial immersion results in

steam explosions. Hold under water until it stops making noise. Casting can then be

fished out. Remaining investemnt 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 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 it quickly. Aluminum metls at a relatively low temeprature 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. 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. 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 (hemihtdrate) 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. 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. Overbeating the slurry will cause

it to cure too fast. Likewise the presence of foreign material, especially flakes of

old investemnt, 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 Session 1

Pour 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

walkthrough 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. Deold the parts and let them keep their

castings.

Finish spruing and invest the patterns 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. For small

patterns this is less imporatnt becuse 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 the first time. 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. If using a cordless mixer, then only mix for about a minute and don't sweat

lumps in the mixture. Too much imixing 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 hold under

vacuum for about 1 minute, or until bubbles start to subside. Pour the investemnt down the side of the flask, not over the pattern. You

want to have the pattern get covered by a gradually rising surface of investemnt

without a lot of splashing. 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 repeared 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.

END OF FIRST SESSION

Before 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. 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. You should also 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 undertand

how to review the settings and make changes as needed. Finally, you can discuss the students particular projects. For the instructor

this is the most interesting and rewarding part of the the class.