DIY investment making

[Derek Courtis]

Does anyone here have any experience making investment powder from separate ingredients?

I'm thinking of massively scaling up my current production level from individual pieces to casting big trees inside large flasks.
But I'm wanting to keep costs down as I go into it.
The commercial investment that I'm currently using costs $7 per kilogram.
And it looks like I can make it myself for as little as $3 per kilogram
[Note: these prices are AUD]
While $7 per kilogram is nothing when using tiny flasks, IMO that'll become a significant expense when doing large scale casting.
And when scaled up that's a massive potential saving to be had.

I had a quick look online and from what I can tell the basic receipe is 50% plaster with 10-50% silica.
Some people suggest to add something extra for refactory such as olivine sand or powdered fireclay.
In my case I'll probably also need to add some borax to prevent breakage during burnout since I'm using Senertek casting resin (which has a tendency to expand during burnout).

The way forward seems pretty clear...just have a go and see.
Only putting this out to the forum in case anyone has some experience or wisdom to share.

[Derek Courtis]

[quote="David" pid="279" dateline="1701318590"]
I don't but I'm very interested in learning about your experience.
[/quote]

I was afraid of that response....lol
Then in case I'll be sure to post my results up here after I get around to giving it a go.

[Javier Odom]

I have the same response.

However, your post has piqued my curiosity on the topic.

A quick look at this... (btw I am not going to trim weight figures)


Currency
When I ask Google what the USD to AUD is, it says 1 USD = 1.51 AUD.


RioGrande
Rio currently sells a 44 lbs bag of Ransom & Randolph® Plasticast® Investment for approximately $56.50 USD if you are buying one box.  Their site lists the best discount is if you purchase 24 or more boxes at $52.00 USD.

44 lbs / 2.2026 = 19.97639153727413 kg

56.60 USD * 1.51 = 85.47 AUD

85.47 AUD / 19.97639153727413 kg = 4.2785505 AUD per kilogram


SHIPPING
Does Rio ship to Australia?  Is shipping what significantly changes this (approximate cost of 2.72 AUD per kilogram)?


IF you bought bulk...
52.USD * 1.51 = 78.52 AUD

78.52 AUD / 19.97639153727413 kg = 3.93 AUD per kg

Shipping for (24 * 19.97639153727413 kg) 479.4333968945791 kg would probably cost a pretty penny, but I would guess you may be able to negotiate a lower shipping rate for this, which would drop that ~2.72 AUD per kg cost.

All of that, and you are still not at the $3 AUD per kg you want to be at.  However, how much are you paying for your $3 AUD per kg supplies, including shipping?  Still under $3 AUD?


Localized Options
I do not know how shipping costs work in Australia, but I did find a company that sells an investment for $61.60 AUD for a 25 kg bag.  I also do not know if this is even the correct investment.  I am looking for options here.  That is approximately 2.46 AUD per kg.  If shipping is more manageable for this, that may get you closer to your $3 AUD per kg target and you would have a smaller ecological footprint (not shipping 479.4333968945791 kg + plus the pallet(s) and bindings across the planet), and you would be supporting an Australian company.

This was for a single bag.  If you are doing a lot of casting, then you may be able to work out a better price with them.

https://www.cjservice.com.au/satin-cast-20-sc20-investment
(I do not know this company, I just found them in a +/-30 second google search for https://www.google.com/search?q=australian+jewelry+casting+investment.)


My Thoughts
I would find a better price from an external vendor than your $7 per kg first.  If the investment formula needs tweaking, I would check in with the manufacturer of the investment to see if they would be willing to make a new recipe.  If they would do that, it would help you and them as they could market the product to other jewelers.  This would allow you to focus on making jewelry.

If you cannot find a lower priced alternative and you still need to make investment, then let's see about making "open source jewelry casting investment"...


General Ingredients

• cristobalite
• gypsum
• silicas
• modifying agents

I have also seen the following ingredients listed by Richard D. Austin:

• cristobalite
• gypsum
• quartz
• wetting, defoaming, and reducing agents
• fibers

From where are you getting your recipe and instructions?

Also, are you vacuum casting or otherwise?


General Reading
This is quickly going to get into chemistry meets casting shop ... perhaps practical chemistry.

https://patents.google.com/patent/US6013125A/en

66th_annual_tech_conference_and_expo_book.pdf

This is going to be an interesting project!

[rosemary]

Yes please.  A casting investment recipe would be helpful!

[Javier Odom]

I am curious where you are in this project as well. If you want anyone to help you experiment let me know. I can set some time aside to try some different formulas and test them.

[rjt]

i've never made my own but I'v used investment someone else made for us.

[Javier Odom]

I found more information related to your question. I have not tried this yet, as I am working on a few other projects, but want to post this before I lose the information. I would be very interested in results from this, or from your own experimentations.

This is a venture into the alchemy of investment casting, a path where chemistry and metallurgy converge, and where cost savings are measured not only in currency but in control over the process itself. Let us proceed ...

The Essential Ingredients

Investment powder, at its core, is a carefully balanced blend of binders, refractory materials, and modifying agents. Each component serves a distinct purpose, and their proportions must be considered with precision.

1. Binder (Plaster of Paris or Gypsum – Calcium Sulfate Hemihydrate) (40–50%)

• The backbone of the investment, responsible for holding the structure together.
• When mixed with water, it forms calcium sulfate dihydrate, which hardens into a strong yet porous matrix.
• High-quality dental or industrial-grade gypsum is preferable.

2. Refractory Material (Cristobalite and/or Quartz Silica) (30–50%)

• Provides the necessary heat resistance, preventing thermal expansion from compromising the mold.
• Cristobalite is preferred due to its controlled expansion properties, reducing stress fractures.
• Quartz silica can be used as an alternative or supplement, but it expands unpredictably at high temperatures.

3. Modifiers and Additives

• Boric Acid or Borax (1–3%) – Helps prevent investment breakdown during burnout, particularly important for resin-based castings.
• Olivine Sand (optional, 5–10%) – Enhances refractoriness and reduces thermal shock.
• Fireclay (optional, 5–10%) – Further stabilizes the structure, particularly for larger castings.
• Wetting Agents (trace amounts, <1%) – Reduce surface tension, improving mold flow. Common options include sodium lauryl sulfate.
• Defoaming Agents (trace amounts, <1%) – Minimize air bubbles, ensuring a smoother surface. A few drops of isopropyl alcohol can serve this purpose.

The ratios of these ingredients will vary depending on flask size, burnout temperature, and the type of metal being cast. As with this entire process is at this time, experimentation is inevitable.

For the preparation/mixing, you will need distilled water, and various “tools”.

The Method of Preparation

1. Sifting and Mixing

• Weigh each ingredient precisely.
• Sift all dry ingredients together through a fine mesh to ensure homogeneity.
• Blend in a dry container using a paddle mixer or by hand in small batches. Avoid excessive dust inhalation; silica is not a friend to the lungs.

2. Water Ratio and Mixing Process

• The ideal water-to-investment ratio typically falls between 35–40% by weight.
• For a 1 kg batch, this equates to 350–400 mL of distilled water.
• Add water to a clean mixing container first, then gradually introduce the dry mix while stirring.

3. Vacuum Degassing (Recommended, but Optional)

• If available, place the mixed slurry into a vacuum chamber and pull 28–29 inHg (95–98 kPa) for 60–90 seconds.
• The mixture will expand violently at first, then settle.
• This removes air pockets that could cause surface defects in the final cast.

4. Pouring into Flasks

• Pour the investment slowly along the side of the flask, avoiding turbulence.
• Tap or lightly vibrate the flask to release any remaining trapped air.

5. Setting and Burnout

• Allow the investment to set undisturbed for 2–4 hours before handling.
• Place flasks in the burnout oven and follow a gradual temperature ramp:
  • Room temperature to 200°C (392°F) over 1 hour – Moisture removal.
  • 200°C to 400°C (752°F) over 2 hours – Binder breakdown.
  • 400°C to 750°C (1382°F) over 2–4 hours – Complete burnout.
  • Hold at 750°C for 1–2 hours before casting.

Notes on Scaling Up Production

For large-scale operations, batching investment ahead of time requires airtight storage to prevent premature moisture absorption. Desiccant packs or climate-controlled storage can mitigate humidity issues.

Recap and Outtro

There is no singular "correct" recipe - only optimal formulations that suit specific applications. Your goal should be to refine your mix iteratively, testing different ratios and observing how they perform with your casting setup. Commercial investments are formulated through rigorous testing; your homemade version will require the same level of diligence.

If nothing else, this endeavor will provide a deeper understanding of the very foundation upon which precision casting rests. Whether it becomes your permanent solution or merely an intellectual exercise, you will have gained something invaluable: knowledge.