How Much Jesmonite® Do I Need? Simple Mix Ratio Guide

In a hurry? Use our free interactive Jesmonite® calculator — pop in your mould’s water weight and it gives you the exact liquid and base amounts. Prefer to do it by hand? The simple method is below.

Below you will learn the quick water-volume method, how to turn that figure into Jesmonite® base and Jesmonite® liquid using the AC100 ratio, and a worked example you can copy for coasters, trays and pots.

The quick water-volume method

Jesmonite® is a water-based material, so water is a brilliant stand-in for working out how much to mix. The trick is to measure the water your mould holds, then feed that figure into one simple AC100 sum. Here is how to measure it:

1. Make sure your mould is clean, dry and sitting level.
2. Fill the empty mould right up to where you want the cast to finish.
3. Pour that water into a container on your scales and weigh it in grams. Because 1 ml of water = 1 g, a mould that holds 200 ml gives you 200 g.
4. Add about 10% on top for spare, because some mix always stays behind in the cup. A 200 g reading becomes 220 g.

If you are casting several pieces at once, fill and weigh each mould, then add the totals together. Working in grams keeps everything simple, because the mix ratio is by weight too.

Working out the mix ratio (AC100 1:2.5)

Once you know how much water your mould holds, you can turn that figure into Jesmonite® base powder and Jesmonite® liquid. AC100 mixes at 1 part Jesmonite® liquid to 2.5 parts base powder by weight. Turning your water figure into amounts takes just two small steps:

1. Liquid = your water figure (with the 10% spare) divided by 2
2. Base = that liquid amount multiplied by 2.5

That is the whole sum. Stick to the AC100 ratio of 1 part Jesmonite® liquid to 2.5 parts base, as this is the number that decides whether your cast sets hard or comes out chalky.

A worked example

Say you filled a coaster mould with water and it weighed 200 g. First add about 10% for spare, which brings you to 220 g. Now run the sum:

1. Liquid = 220 / 2 = 110 g
2. Base = 110 x 2.5 = 275 g

That gives you 110 g of Jesmonite® liquid and 275 g of Jesmonite® base, so 385 g of mix in total, with a little spare built in so you are never scraping the bottom of the pot as the material starts to thicken.

Weigh, do not guess. A cheap set of digital scales that reads to 1 g is the single best tool for consistent Jesmonite®. Getting sticky, chalky or fast-setting results is almost always a wrong ratio or inaccurate scales, not the material.

How much to mix for coasters, trays and pots

Every mould is different, so the water test always beats a generic chart. That said, here is a rough sense of scale to sanity-check your own measurement:

• Coasters: shallow and small, so they need a modest batch. A set of four is still a quick mix rather than a big one.
• Trays: wider and often deeper at the walls, so they use noticeably more. Weigh the whole mould, walls included, not just the base.
• Pots, vases and candle holders: these vary the most because of hidden internal volume and thick walls. Always fill with water first, as it is easy to underestimate a chunky pot by eye.

Browse coaster and tray moulds or vase moulds and run the water test on each one before you mix. Most of our own mould product descriptions also list the amount of mix you need, which takes some of the guesswork out. New to casting altogether? An AC100 starter kit pairs the Jesmonite® base and Jesmonite® liquid in matched quantities, which helps with your first few pours.

Tips to avoid waste

Mixing the right amount is half the battle. These habits help you use every gram:

• Batch your pours. If you are making several pieces, weigh all the moulds and mix once. AC100 gives you a comfortable working window of about 10 to 15 minutes (a little less if the room is above 25 °C), so one well-planned batch beats several small ones.
• Keep a maker’s log. Write down the water figure for each mould you own. Next time you skip the measuring and go straight to mixing.
• Add pigment as a share of the total mix. Liquid pigments are powerful, so a little goes a long way: you can add up to about 2% of the total mix weight, and beyond that it can affect how the cast sets. Weigh your Jesmonite® base and Jesmonite® liquid first, then colour.
• Have a small backup mould ready. If you mix slightly too much, pour the leftover into a tiny mould or jewellery shape rather than throwing it away.
• Do not stretch the liquid. Adding extra Jesmonite® liquid to make a batch go further leads to soft, chalky casts. If you are short, mix a fresh small batch to the correct ratio instead.

One more note on planning ahead: AC100 is made for indoor decorative pieces. If you are casting something for the garden, you would switch to an outdoor-rated variant such as AC84 or AC730. Those mix at a different 1:5 ratio, so this AC100 sum does not apply. For those variants, reach for an online Jesmonite® calculator or the Jesmonite® app to work out your quantities instead.

Ready to mix your first batch?

Grab your scales, run the water test and let the AC100 sum do the rest. When you are ready to stock up, explore AC100 Jesmonite® base and Jesmonite® liquid and the full range of silicone moulds at gigacrea, with sharp prices and same-day dispatch on orders placed before 2 pm.