Introduction: Lost Wax Investment Casting of Silver Pendant
This is my follow up of my silicone mold making Instructible. I injected waxes using the mold I made in that Instructible and used one those waxes to make an investment casting in silver. I have skipped over the wax injection process for now (because I forgot to take pictures), but will come back to it next week when I run some more waxes. I will pick up the investment casting process with waxes or whatever other organic material you would like to make a casting of.
This entire process, from preparing or designing and making a master to making molds to injecting waxes to making investment to pouring the metal is tedious, but strangely fun and rewarding. There are a lot of places for things to go wrong, which makes it even more satisfying and exciting when it goes right.
Here are the supplies that I used for the wax investment casting process:
Prepared casting waxes or other organic form to be cast
925 sterling silver casting grain (supplemented with clean silver scraps)
Kerr disclosing wax
2.5” steel casting flask
2.5” rubber base with ¼” hole for sprue
Rubber mixing bowl
Volumetric flask for water
Vacuum assist flask stand, built by Jim
Rubber gasket for vacuum assist
Xacto knife for spruing
Scale that can weigh in grams
Timer on my phone
Steel plate with rods to catch wax
Thermocouple or kiln controls
Small round crucible with holder
Flux (borax powder)
Leather gloves, protective glasses
Bucket with water
Big steel tongs to pick up hot flask
As you can see, this is unfortunately not a project that you can wake up one morning and decide to tackle. It takes a bit of preparation to get everything lined up ahead of time as well as some workshop space. One step at a time, it can totally be done!
Step 1: Step 1: Prepping Waxes
The lost wax method did an absolutely amazing job of copying ALL the detail. In anticipation of this, I tried to get my waxes as perfect as possible. I used Kerr’s disclosing wax with a flimsy metal film as a spatula and my finger to smear it in any creases that were caused by imperfectly filled waxes (folds, faint lines, small imperfections, etc.). Disclosing wax is very fine and can be used sparingly and should all but disappear when applied properly.
I also used a very fine wet/dry sandpaper to smooth the edges where there was minimal flash. I picked it off with my finger, but even a small wax burr left behind would create a sharp silver burr that will need to be dealt with later. Wax is much easier.
I made sure my sprues are relatively neat and clean for a couple of reasons. First, I didn’t want to waste any silver on sprue lumps and second, I wanted to keep the channels as smooth as possible. I have read that roughness can cause turbulent flow which can cause problems in the finished silver castings. I used a chisel and sandpaper to clean them up from the roughness caused by the box.
Step 2: Step 2: Begin Measurements
Silver casting is a little bit more like Chemistry class than art class. Since this stage of making my pendants is much more science than art I heeded the advice my Chemistry professor said over and over in lab; write everything down in a notebook.
--Weight of the sprue base with no wax
--Filled the flask with water and poured in a volumnetric flask to get total volume capacity of steel flask.
--Per Americast instructions multiplied volume of steel flask by .6 to get the amount of water I will need for the investment mix. (the mix will be 60% water and 40% investment)
--Calculate weight of investment needed by multiplying the water ml by 2.5.
Step 3: Step 3: Wax Tree
The less distance that the molten silver needs to travel, the better, so shorten the sprues to be as short as possible while still allowing the necessary space between waxes. There should be at least ¼” between models and ½” between model and sides and ½” to 1” between tree and top. If the top is too thin, the molten metal can bust through during pouring.
We attached the sprues of the parts to the trunk using an Xacto blade heated with the torch touched to both pieces of wax to get them to melt together. I recommend practicing this to get the feel of it on junk pieces of wax before tackling the real thing for newbies. It takes a few times to get the feel.
Make sure the attachment points are solid, smooth, and there are no gaps in the wax. Any gaps would be filled by investment and either break off into your mold during the pour or interfere with the metal flowing into the part. Use melted wax to fill in if necessary.
Note when spruing: make sure your sprues are big enough to feed your pieces. The sprue needs to attach to the thickest part of the wax and needs to be thicker than the thickest part of the wax as well. This is due to the way the molten metal will crystallize from the walls inward. If the sprue crystallizes and solidifies faster than the part (because it’s smaller), it will cut off the flow of metal to the part (which will be a pain in the butt).
The smallest pieces should be toward the top of the tree as you build it and the larger pieces toward the bottom and you should work from the top.
I only poured 3 waxes this time so we just spaced them evenly in one row.
If there are multiple major sections in a single wax part, sprue each major section to ensure proper fill. My pendants were small and even enough that I didn't need to take this extra step.
You should try to maintain a 45 degree angle of your sprues to the tree. This allows the metal to flow more naturally and smoothly down the tree when you have it upside down for the pour. Our tree was small enough that this wasn't crucial.
Step 4: Step 4: Weigh the Tree
Science time! In the notebook:
--Weigh the tree on base
--Subtract base weight previously written down from total weight to get wax weight
--Multiply wax weight by 10.4 to get silver needed but then ADD an amount (add up to 50% of the amount needed for wax) for button.
Step 5: Step 5: Assemble the Flask
Carefully put the flask on the rubber base and put a piece of masking tape around the top of the flask. This will prevent overflow when de-gassing.
Step 6: Step 6: Investment Mixing
This is a kind of stressful step! You have to finish in a window of 7.5-9 minutes. If you finish too soon, the investment will separate and if you finish too late, it will start to set up.
Pour the water amount you previously wrote down into the rubber mixing bowl. Deionized water is recommended by the manufacturer. Deionized or distilled is recommended because it is more stable and the plaster slurry is more likely to have predictable properties when mixed. Extra ions or substances in the water can adjust the set up times.
Add the investment to the water and mix out the lumps. I used my hand so that I could feel lumps and crush them. It is the consistency of thin pancake batter.
I started my timer when I started adding investment. I allowed 3 minutes for mixing.
Step 7: Step 7: De-gassing Investment
Place the rubber slurry bowl in the vacuum chamber and don’t freak out when it starts to vigorously boil. Let it boil it’s little heart out for about 30 seconds. Then turn off vacuum and pull the pre-de-gassed mixture out of the chamber. Get ready to pour.
Step 8: Step 8: Pouring Investment
Pour the investment down the side of the flask, being careful to not pour crazy and add any air in the process. Also, don’t pour directly on the wax tree. The weight of the investment may break the parts or trap air. Using the volume method I described earlier should give you just about the right amount of slurry to fill your flask with little to none leftover.
Step 9: Step 9: Re-de-gassing Investment
Now put the investment filled flask back in the vacuum chamber and let it boil for 60-90 seconds. It spits and seems like it should not be doing this. It’s ok.
Once the time is up, let it set up for about half an hour and then remove the sprue base. At this point, let it air dry anywhere from an hour to overnight. There is some wiggle room with this step.
Step 10: Step 10: Wax Burn Out.
I followed the instructions/schedule from Americast for burning the wax out. I placed the flask with wax side down on a couple of steel rods to allow the wax a place to melt out. Since I don't have a controller for the kiln, I had to maintain temperatures manually. I kept my little notebook next to me for writing down temperatures and times so could make sure I wasn’t heating up too fast. 9 degrees per hour is a reasonable ramp time.
Follow the schedule that comes with your investment! You need to start slow or else the wax will expand early on. The temp rises to cure the investment once the wax is melted out.
The last hour is a heat soak because we wanted the flask to be about 1000 degrees when we poured the silver.
Step 11: Step 11: Weigh the Silver
Get the number you calculated earlier from your notebook.
Silver needs to be at least 50% virgin. We used 60% virgin and 40% clean scraps.
While you are at it, get vacuum table (or centrigual getup) ready and bucket of water ready
Step 12: Step 12: Begin Melting Silver
Use a torch and keep it on the melt to keep the oxygen out and therefore the oxidation down. Get it mostly melted before removing flask from heat soak in kiln. Depending on how much silver you have and what power or torch you have, it may take a while for the silver to melt. Add a little borax to the melt.
Turn vacuum table on, if that is what you are using. Place rubber gasket down on vacuum table.
I will get more pictures my next melt, but my hands were full and I forgot to take sufficient pictures of the melt and pour.
I torched the silver, keeping the torch on the silver but moving around in circles to keep the heat even on the surface of the silver. When it first melts, the surface tension keeps the silver in a ball. I poured when the surface tension just barely broke since the pour temperature is almost 100 degrees Celsius higher than the melting temperature.Va
Step 13: Step 13: Pull Out Flask and Pour
Use tongs to pull the flask out and place it hole up on the vacuum table gasket. Keep torching the molten silver until it loses it's surface tension then pour into the hole, keeping the flame over the metal the entire time. You want the flame to a reducing flame (low on oxygen) to keep oxidation from occurring.
Pour fast and pour confidently. The longer it takes you, the more risk there is of the metal crystallizing up and blocking flow before you are done. Just git 'er done.
After you are done pouring, let the metal rest for a few minutes before quenching.
I have no pictures of the pour. Please forgive me. I will update with pictures soon.
Step 14: Step 14: Quench in Water Bucket
After a few minutes, using tongs, dip entire flask under water to break the investment. Fish out the silver from the bottom of the water bucket after the bubbles stop.
Forgot pictures--I will add next time.
Step 15: Step 15: Brush and Pickle
It came out with some oxidation that was easily removed with scotch brite and a warm pickle. Noooo, not that kind of pickle. Pickle is an acid solution that removes the firescale and other crap on the surface.
I used battery acid (sulfuric acid) diluted to 10% heated in a hot water bath of about 150 degrees F for about 20 minutes. In order to keep me from having to put my hands in battery acid, I affixed a thin silver wire to each pendant and placed them into a glass jar. Cleans up nicely.
Step 16: Step 16: Finish
We dipped the piece in warm liver of sulfur and then buffed to add a little strategic oxidation.
I hope this helps, and as I mentioned, I will be adding more pictures with my next pour. Please feel free to contact me or comment if you have questions or comments. I love Instructables, both learning from others and sharing what I have learned and I hope to have helped!!