Investment mix - Hardening time

Hey guys…

Ive been wondering about it for quite a while now since I wanna optimize my time when doing lost wax casting.

When i’ve mixed, vacuumed etc etc. I usually let my mold sit for 1.5 - 2 hours before putting it in my kiln to do the burnout…

However, is there a limit to how long is prefferable to let my mold sit? Can i leave it over night and start the burnout next morning.

Heard that it’s good to have the moisture in the investment since it helps to push out the wax, and it has therefore been my concern if its bad to let it harden too long if all the moisture dissapears?

Best Regards William

Hi William, that is my usual way of casting. I invest in the evening, usually after a long day of preparing waxes, and start the burnout the next morning.

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You can/should leave it overnight. Having moisture in the investment does indeed push the wax out but it also leaves a more grainy texture and adds nothing of value to the burnout.

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Sounds good, Thank you so much for the reply! This really gives me a lot of freedom that I dident have before! :hear_no_evil:

Thank you so much for sharing, and even adding additional knowledge to my question! I’ve been experience some slight grainy texture on my previous cast.s

Actually just casted my best casting so far, which i probably owe to letting the mold sit over night!

Soo stoked!

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Thanks much for these posts. I was also taught to let the invested flask sit for exactly 2 hours (and that’s consistent with the investment instructions as well) before putting the flask in the kiln. It’s a killer because it stretches my casting days an extra 2.5 hours longer. I will definitely try this!

Not trying to be contentious but without knowing what the burnout ramp and soak cycle is being used it is hard to make an informed statement. I normally use an overnight very slow burnout cycle regardless of how long the canister has set after investing.

I can believe the drying time would make a difference if a constant temp burnout is employed without a ramp and soak cycle.
Regards RLW

I always thought some of the concern about letting the flask dry out is that the investment will start to crack internally and you might see finning on some of your pieces.

I usually prefer to have it sit for 4-6 hours before I cast and like others, invest in the evening/night and burn out in the morning so I can cast by the following evening.

Hey Rwade1

Can you perhaps elaborate a bit on this ?

Example of your soak cycle etc if you put in an investment after 2 hours hardening time versus if you are having a flask out for say 8 hours?

If its only been sitting for 2 hours, would you the ramp it to arnd 150 degrees C for a couple of hours before raising the temp?

Whereas if you let it dry for longer you can put it in a hotter kiln ?

Just had Peter Johns (Inventor of Argentium silver tell me following), after i had exactly what @arkaysilversmiths refered to.

“What happens is the outside of the investment dries first when it is allowed to stand for a long time in air. When the outside dries it seals the outside surface of the investment. This then traps the moisture inside. When the heat comes on, the water inside is turned to steam and the steam cannot escape. The pressure of the steam cracks the mould. Only allow the investment to reach its green strength about two hours and then start heating. This is where the 150°C is important and it should be done for several hours. It draws the moisture out slowly from the middle of the flask without sealing the exterior surface. I think the roughness of you casting is also caused by the water boiling inside the mould and roughening the surface of the investment.”

Best regard William


An interesting observation I made with leaving a 2 1/2" x 3" flask overnight after investing, I took the rubber base off the end of the flask and put the flask on a ceramic shelf with the original exposed end of the flask down for about 10 minutes. When I came back, there was a damp spot under the flask, so even after sitting out in the open for 8+ hours, the investment is still ‘damp’ even at the end that was exposed to the air.

I’ve started to run my burnout with the first 2-3 hours at 300°F while removing the sprue wax. When I removed the flask from the kiln to get the wax tray out, the surface of the investment was nice and dry, no wax residue to bubbling water residue around the funnel section. Other times if I only run it for an hour, there is still water vapor moving through the investment and there might be residue still on the mouth of the flask from the water boiling off.


Have also tried having my flasks sit overnight with my “homemade” extender made of baking paper and tape + rubber buttom on…

The next day when i remove it to put in the oven, I can still feel that the invesment is a bit moist!

Have begun to give my moulds longer hold time at 150C to eleminate water & wax more slowly!

After removing the rubber base we always lay our flasks down on their sides with the bottom exposed. We usually leave them out over night.
We do not do rush jobs any more, but back in the day if we needed to we’d always leave the flask out on it’s side for at least 2 hours. Attached find a link to Stuller’s guide to investing and burnout.

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