"Steam Curing" in a black plastic bag works very well. No need to add wicked water...just put as much as you can inside the bag, then seal it up as air tight as possible and the moisture evaporating from the mix will re-cycle itself with the suns help and maintain a constant high-humidity envionment. Keep it sealed up for at least 72 hours and you should be OK. I routinely cure "neat Portland" mixtures this way and they are without a doubt the trickiest and most demanding mixes to cure. Note...if you search online you can find 5 foot tall by 3 foot wide black plastic trash bags from industrial supply houses...great for larger works.

Thanks tango88. I've got it in the bag as you recommended. Now I'm wondering if I can put another, smaller one in the bag too, maybe on top of the large one. Any thoughts?

Not sure what you mean. Are you talking about setting one uncured piece on top of another? If so, better put a sheet of plastic between them so they don't become one...other than that, just add more water into the bag after you open it as you will lose quite a bit by simply unsealing it.

Another trick is to set the work you are curing up on a couple of bricks inside the bag, then fill the entire bottom of the bag with water. That way you can create a large water supply without the piece actually sitting in it, which would alter the way the submerged section would cure. While you can totally immerse concrete and achieve an excellent cure, you must wait until the initial set is complete or you will simply be adding more water to the mix and weakening the final product. "Steam" curing in a bag allows you start the process much sooner and avoid early onset shrinkage cracks that often still occur with immersion curing.

I have found that either totally immerse the piece or have it completely above the water. Especially if you aren't painting it.
You can get some persistent efflorescence patterns if you half submerge your piece.

Even contact with a wet cloth can create patterns that can be difficult to remove. It could be my local water creating this.

I has stopped submerging most of my work, especially the dark colored stuff opting instead for the steamy tent.

At the moment I'm steaming a small bowl with a Portland 'neat' finish accented with copper filings. I'm excited to see how the color ages on this almost black piece.

tufaenough and/or tango88 -- I'm not familiar with Portland neat finish. Could you explain what that term means? Why do you use it or what is it's purpose?

Why do you advise heat? If curing concrete at more than 90F is not recommended, why would you want to put a piece in a black plastic bag in the sun?

As far as I've ever seen, heat+concrete= cracks. What am I missing?

Sue

Hi Fleur
Tango's definition may differ but 'Neat' is Portland cement with no added aggregates of any kind.
Additives, admixes and colorants are OK.

Here are a few links

Here is a link that might be useful: Steam curing

As Tufaenough stated, "Neat" Portland (cement) is defined as cement that is mixed without aggregates. Traditionally, neat Portland (nothing but Portland & water) has been used by masons as a bonding agent when building up additional layers onto existing or previously cured concrete. While it lacks the tensile strength of conventional concrete, it is extraordinarily dense and has tremendous resistance to compresional forces. Properly cured, it results in a surface that is virtually impervious to water and can be finished to a surface that is similar to fine china. It is, however, extremely difficult to achieve a proper cure and it can be very unforgiving. Fracturing is the biggest problem resulting from a less than "perfect" cure and can range from micro-cracks to total failure from fissures that cause the work to fall apart. So why do it if it is so demanding???...the finishing possibilities. With no aggregate to get in the way, it can be worked in detail that equals the best sculpting clays.

As for the "Heat" issue...the "90 degree rule" is designed for conventionally cured work and meant to represent the point at which dehydration occurs so rapidly that it becomes almost impossible to control. Of greater concern is air movement over the work. It will remove moisture even more rapidly than heat and is a much more common cause of failure. Even a "slight" breeze carries moisture away at a rate that can result in major problems. And this is particularly true for 3-dimensional pieces like sculpture, where more of the surface is exposed to the air. If you are pouring a sidewalk, only the top surface is exposed...but a sculpture often has more than 90% of it's surface area exposed and losing hydration into the surrounding air. So, even though it's tempting on a hot summer's day...DO NOT WORK WITH A FAN BLOWING ON YOU AND YOUR WORK! Keep the breezes off your work and you will be helping to keep moisture in.