A casting studio in Ahmedabad switched investment powder brands and, without changing the water ratio, started producing rings with a rough, orange-peel surface inside the shank. The new powder needed a slighter thicker mix. At the old water-to-powder ratio, the slurry was too thin, the investment had low green strength, and the hot metal scoured the mold wall as it poured. Investment ratio is not a fixed number on a bag; it is a recipe tuned to alloy, flask size, and burnout schedule. This guide covers the typical ratios and how to read a bad mix.

Investment (plaster-bonded silica, gypsum-bonded for silver and gold) is mixed with water under vacuum to form a slurry around the wax tree. The ratio controls strength, expansion, surface detail, and permeability. Too much water: weak mold, grainy surface, expansion that cracks. Too little water: thick slurry that traps air, incomplete detail, and a mold that breaks out during burnout. Standard ratios for jewelry investment run roughly 38-45 parts water by weight per 100 parts powder, depending on brand.

Typical ratios by alloy and use

- Sterling silver (lower temp, 760-800 C burnout): about 40-42 ml water per 100 g powder. Silver tolerates a slightly thinner mix.
- 10k/14k/18k gold (higher temp, 650-750 C burnout): about 38-40 ml per 100 g. A firmer mix resists higher pouring heat.
- Platinum or high-temperature alloys use phosphate-bonded investments at different ratios and are outside this guide.

These are by weight, not volume. A cup of powder varies 20% depending on packing, which is why bench scales matter. The bag always prints a starting ratio; the caster tunes it 1-2 ml either way based on flask size and surface result. For material reference, see gold alloy basics.

Mixing under vacuum

The powder is slowly sifted into weighed water while a vacuum mixer stirs. The slurry is mixed for 60-90 seconds, then placed under full vacuum (about 28-29 inches of Hg) for 30-60 seconds to pull out trapped air. A mix that is not properly vacuumed leaves pinholes on the casting surface. The flask is then filled in a slow, steady stream down the side of the sprue, never poured directly onto the wax tree, to avoid tearing wax or trapping air.

Set time (the investment reaches green strength) is about 45-90 minutes depending on room temperature. A flask set in 20 C shop temperature behaves differently from one in 32 C summer heat; experienced shops note the room temp on the casting ticket. Humidity matters too: a damp storage area for powder absorbs water from the air and throws off the weighed ratio by a few percent, which is why sealed bins and dated powder bags are standard in a consistent foundry. A flask set in 20 C shop temperature behaves differently from one in 32 C summer heat; experienced shops note the room temp on the casting ticket. See also spruing practice and CAD wax patterns.

Reading a bad mix from the casting

- Orange-peel rough surface on the inside of rings: mix too thin or investment over-aged.
- Pinholes all over the casting: incomplete vacuum on the slurry.
- Investment stuck hard in crevices and hard to blast: mix too thick or burnout temperature too low.
- Mold cracks in the flask after pouring: expansion too high, usually from too much water or too high a burnout ramp.

What is the standard investment ratio for silver jewelry?

About 40-42 ml of water per 100 g of powder by weight. Gold typically uses 38-40 ml per 100 g.

Do I measure water by volume or weight?

By weight on a bench scale. Volume measures vary 20% with packing density and cause inconsistent molds.

Why did my surface turn orange-peel after switching powder brands?

The new brand needs a thicker mix. The old ratio left the slurry too thin, lowering green strength so hot metal scoured the mold wall.

How long do I vacuum the slurry?

Mix 60-90 seconds, then hold full vacuum for 30-60 seconds until bubbles subside. Skipping vacuum leaves pinholes on the casting.

Investment ratio is one of the quiet variables that separate a consistent casting shop from a lottery

Why the mixing ticket matters on reorders

Investment ratio is a process detail that drifts between mixing days. A caster who mixes by feel on Monday and by scale on Friday produces molds that age differently. The fix is a written mixing ticket per flask: powder batch, water weight, room temperature, vacuum time. On a reorder, the ticket lets the buyer compare the new lot against the first. A factory that cannot produce a ticket is mixing by habit, and habit is what produced the orange-peel surface on the powder-brand swap.

. We weigh every slurry, log room temperature, and tune ratio per alloy on the casting ticket. If you are auditing a foundry, ask to see a recent mixing ticket; the ratio should be in writing, not in the caster's head. Use the supplier audit checklist to line up your next lot.