Strange, that shouldn't be happening. If you don't get an answer here, try asking on the Hypertufa forum.

Finally something I'm familiar with!
First: I suspect your cement:sand ratio is off. I'm not an expert but it seems to me that you have too much cement (or not enough sand) in the mix. This could result in an easily eroded finish. Perhaps try at least doubling the amount of sand. I assume you're measuring the concrete ingredients by volume?
Second: Was the concrete fully cured when you tried to wash it? Remember concrete has to cure, it doesn't dry like plaster.
Third: I never, ever try to brush or wash off leaf remnants from my leaf castings. It's too easy to ruin the detailing. If you set the cured casting outside the leaf remnants will weather off on their own thus preserving the leaf impression.
And last: In the Fall the sugar content in the leaves of some plants greatly increases. This higher sugar content can effect the curing process of the concrete resulting in holes, rough surfaces and/or powdery finishes. There's nothing you can do it about, it's nature.
This sugar effect is often used in the concrete industry to add "age" to cast items. Sugar can be sprinkled in/on the mold or actually mixed in with the concrete to intentionally cause "bubbles" to form. These bubbles will cause the cured concrete to flake and erode thus resulting in the impression of age. Be aware that mixing sugar in with the concrete can result in a weaker finished product because of the bubbles or pockets produced.
Hope something here helps. Let us know how any future castings turn out. Have you tried using vinyl patch cement for leaf casting? Talk about preserving detail! WOW! And the finished casting is lighter in weight than a regular mortar one.

I suspected the ratio as well and tried a 3:1 ratio (increased the sand)on my newest casting. The quikrete worked really well for me but I sifted out the large gravel pieces so I would have something smoother to work with. I've since been advised I shouldn't take out the gravel...oops! I thought I would try the portland/sand mixture because it would save me sifting work. I want to make sure what I use is extra sturdy because I'm making birdbaths and don't want them to break, crumble or leak. Would the vinyl patch cement work for this type of project? Thanks for the advice. I'd post a picture but haven't figured out how to yet!

blackcrowe,

I have made many leaf castings and sometimes used quikrete until discovering how much better my leaves came out using vinyl patch. A much smoother and stronger end result. If course, it is much more expensive than using quikrete, but no worry about the ratio's. Everything that nmgirl mentioned about the sugar is true too. i would love to see a picture of what you are doing with making birdbaths.

Where are you in Iowa? I am from the NE Corner, about 75 miles south of Rochester, NM or 35 miles north of Waterloo. I don't know very many other artist who do leaf castings. Did meet several this past summer-Loved discussing "crete" with others!

Becky

Hi blackcrowe,
If I understand what you wrote in your last post it sounds as if you've been using Quikcrete concrete mix for your leaf castings. Isn't it kind of...."chunky" for leaf casting?
I use either mortar mix or vinyl patch. Just in case you're not familiar with it, mortar mix is a cement based product usually used for brick/cinder block laying. It doesn't have the coarse aggregates (gravel) the concrete mix contains. It's pretty much just sand and cement. It's good for leafcastings, stepping stones and I use it to make "millstones". It's easily carved before it cures.
Vinyl patch works great for leafcastings. It's strong, lighter in weight and really picks up leaf details. It's also more resistant to the flaking and cracking associated with traditional cement mixes. That being said I still don't expect it to stand up to winter freezing. I empty my vinyl patch birdbath when freezing weather is expected.
Give it and a bag of mortar mix a try and let us know how things turn out!

Hey becky, what else do you make with 'crete?
Anyone else here like to play with concrete? It's a blast!

I enjoy making things for my garden from concrete. I've used all the mixes you have discussed and also used Sand Topping Mix. One tip--if you use acrylic paint accept that it doesn't hold up too. The colors fade in the sun. I painted some of mine with penetrating wood stain and also used metallic spray paint to imitate bronze on others.

Here is a link that might be useful: concrete garden art

Ladies - Your questions and answers are very helpful. This is something I have wanted to try to tackle (for my own gardens).

suezn - your work is awesome! Thanks for sharing the pics.

Iowa ladies - Do either one of you do the Greene, Ia. arts, crafts and flowers day in the summer? I purchased a couple leaves there this summer and also have purchased from "Country Roads" at Fairbank.

Awesome projects! Good luck! Thanks for the info! Jeanne S.