Mark, Your project sounds interesting. More info would help. Such as, How big and deep do want/need to make your "slabs". Hauling in AND out enough sand to do that sounds like a lot of work. Maybe pieces of styrofoam cut to shape and depth would be easier to work with. Good luck.

Mark... sounds like a great project! I would suggest measuring the dry ingredients for each group that is participating. Then each group can just add water and mix. Sand casting is fun to do but it might be a tad less work if you carve some hard styrofoam like Rick suggested. Use the pink or the blue foam. If you cover the foam with a sheet of plastic it will pop the form out easier. Then again if you find a plastic bowl or platter that is the size you want you can use that for your form too. The sky is the limit with what forms you use.

Have fun and PLEASE take pictures and pop back here and show us what y'all did!

Marly

If you are making short and long a plastic platter will work great. Thats what I have used before. Actually anything lined with plastic as Marly suggested will work good. Anything makes a mold!
Paws

From my experience, you should be aware that new concrete will harm / kill bonsai plants which are sensitive to acidity / PH balance. Concrete needs an extensive period to cure and stabalize before you introduce plant life.

In general 1 year of sitting outside will be enough for the concrete to stabalize. Another option is to fill the planter with water for 30 days adding a few drops of muriatic acid to the water every 2-3 days. Swimming pool testing supplies will allow you to monitor the acidity, PH balance and total alkalinity of the water in the planter. If you were to fill the planter with clean water and let it sit for 3 days without treatment and then test it, you will notice a leathal (to plant life) surge in the total alkalinity.

My wife (bonzai expert) learned this the hard way using a planter that I had built that poisoned the plants.

Markp6,
I like the use of sand casting especially when you have to leave your work and may not get back to it to mist it. The damp sand allows for the project to soak up water when needed. You can enhance this by coving your work with a damp cloth and plastic if you are not able to come back to it.
sounds like a good project.
You can also use dipped fabric layed over damp sand to create your 'slab'. Dip the thoroughly dampened cloth into a slurry mix of 1 part cement and 2or 3 parts sand, mixed with water and a bit of bonding agent. Make sure the cloth is saturated with the mix then drape and fold as you want to create crevices and strata for your bonsai slab.
Dena

We still don't know the size slabs you're hoping to build but have you considered using deli take-out trays or styrofoam meat trays for the molds? You could support them easily with many different things and if each participant brought along a large cookie sheet or tray, they could readily be transported to their homes.

Did I just see Coastal Concepts? What a nice surprise!

MarkP- My question is more about the workshop than the technique. How many people do you think will be attending? Do you hope for all of them to leave with a finished piece? Will you be working indoors? (I assume so in Ohio in January but.....) Will one person be collecting all the supplies and delivering them to the workspace prior to the meeting? How much time will you have to do the presentation and the work?
I think answers to some of these questions will determine which of the many ideas you could use.
Laura

I took a class to make slabs for bonsai or water features and the slab was just that, a rock slab not a bowl. We made an armature of wire and covered it with tulle. Then applied a slurry of wet mortar-like mix over a period of two days. You made small depressions in the surface for planting your trees. Are you making a low shallow planter or a slab?
I plant my bonsai in planters that are 1 to 1 1/2 weeks old. I've been doing it for years. Hopefully this image will post here. This is a low shallow planter with three trees. Billie [IMG]http://img.photobucket.com/albums/v131/Billieann/MVC-010F.jpg[/IMG]

...and you didn't have a problem with the plants being killed? Did you put any kind of sealant on the finished planter?

Here's my experience over 6 months - for what it's worth:
I cure planters for 28 days in a plastic bag. Leaf bags will hold a lot of planters. Then I fill each planter with water, changing every day. We tested the ph: At day 1, it was 10 (alkaline). At day 7, it was a neutral 7. This coincided with no faint scum on the water. Does it stay 7? I haven't checked. But I will.

I would also pre-mix the dry ingredients in sturdy kitchen-catcher type bags. By flopping the bag around and holding it closed you can do a passable job of mixing while containing the dust (don't breath that dust).
As for forms, although I haven't tried it, I like the idea of outside forms of construction foam held together with super-sticky red Tuck tape, or pushing nails into the cormers. Lining with plastic, if you leave wrinkles, will give a nice rustic texture. Wire brushing upon unmolding (when a fingernail will just leave a mark) will take off the concrete skin and make an interesting texture.

Mercedes

People have been making hypertufa bonsai pots for at least 15 yrs, maybe longer. I've put fish in my fountain resevoirs without any trouble (after thorough rinsing).

Markp, if you're in Cleveland, we've probably met. I actually offered to do a demo at one of the few Cleveland bonsai meetings I attended, but there didn't appear to be any interest. I just moved from there, but I'd be happy to help in any way I can.

Justin

Runningdog, I've had no problem with annuals, perennials, trees or shrubs. I usually unmold the day after making. Wirebrush or carve and clean it up. Spray with a hose and put back in the plastic bags for a couple more days. Then place it in a waterbath (change water daily) for 2 or 3 days. I don't seal my planters or mini bog gardens. I'm going on 16 years making planters and stuff. The only thing I have a problem with is posting (and taking) pictures. I'll try that image again.
{{gwi:67665}}

Billie_ann, That looks fantastic! I hope you take more pics...I'd like to see the bog gardens. That sounds like something I've been wanting to try for a while now.

Justin

This is a very elegant shape.... Was it also made with the wire armature and tulle like the slabs you mentioned?
Laura

Markp6, Have tables set up that aren't too tall, unless everyone in your group is tall. The first class I taught the tables were too tall to work on. Mortar pans are great for mixing. Have a printed handout for everyone with all the instructions. I mix in the mortar pans but transfer the mix to smaller containers for the top of table so it's convenient. Along with premixing your dry ingredients have spare rubber gloves (someone always forgets them) and dry cleaner bags to cover your slabs when your finished. Have everyone bring a plastic bucket. You half fill them with water before hand for rinsing gloves and tools, then use that water to wash out your mortar pans. Actually rinse the pans as soon as your done with them or you'll have to chip the dried mortar/concrete/hypertufa off.
Justin, I'm a lousy photographer. It's hard to tell but that's the start of my mini forrest. There are 3 trees in there. Here's a mini bog. The bowl is made from hypertufa. Make sure you smooth the inside of the bowl to get the cement to rise to the surface. This way you don't have to seal. I've never had one leak.
Laura, The slabs we learned to make are just that, a slab of rock with small recesses for plants. I'm trying to find the pics of that. This was made from a large shallow plastic bowl from the dollar store. I also use the lids from plastic lettuce crispers for shallow bonsai planters.
{{gwi:71909}}

Another mold to consider:gold pans.They're either metal or plastic.Very similar to the shapes shown here!
Cheers from here

I thank everyone for their imput and suggestions.
Since the workshop will take place in Central Ohio in January, we are planning on an indoor space, but if the weather allows we could go outdoors. Out meetings are usually held at Franklin Park Conservatory, so an outdoor area should be available if we can use it.
At this point there are 20+ people signed up for this workshop, with the possibility of adding more.
We had not really looked at limiting the workshop to one project-or one size/shape/type of mold. Participants wold be rquired to bring a tray/board or other support to work on top of so that their project would be somewhat portable once it was shaped. Curing would be done at each person's home.

I held a small workshop and what I did was precast a bowl/pot for each person in the workshop the day before.
Then after they finished casting a bowl/pot themselves they could uncast the one's I made the night before and work on finishing those.
This way they got the entire casting AND (more important in my view)uncasting and wire brush finishing all in a single seamless lesson.

They uncasted their second bowls at home the next day without any problems and ended up with 2 different projects for their money.

I have had several requests to do it again but the stress of this first workshop was so great I'm not sure I've recovered enough to do another.:)
Looking back it was fun and the ladies seemed very happy with the result.

Markp6, I limit the size of the class because you don't want to be mixing and cleaning up all day. I let people bring whatever mold they wanted to one of my first classes and a lot of people brought really big molds for troughs. We were mixing hypertufa all day. So I supply the molds now.
I also took a class where you brought a 14" square of plywood to work on and most people couldn't fit them, with the finished project on top, in their cars. Even the trunk can be a problem if people carpool and don't even think about leaning over and putting a wet, heavy piece of cement in the trunk.
It's so much easier to work inside a plastic mold for first time projects. You can throw it in a plastic bag and you don't get wet cement inside the car.
There are some people that get the concept right away and others that you have to help all the way through the project. Even though I tell people that the sides should be 1" or 2" thick you'll see some people with 3" thick sides. I make a couple of pieces the day before so I can show people how to unmold, wire brush, clean up and put back in the plastic bag. I show how to do one then let other people in the class try. The object of the class should be to teach them how to do it; so it doesn't have to be the exact container they want. Some people will never try it after the class due to the weight of a standard bag of cement. For some classes we prebag the dry ingredients and sell them after the class. Billie

20 + people sure equal a lot of supplies to get ready! You might consider a small group approach where everyone helps and learns on a limited number of projects. The finished pieces could later be auctioned off or sold at one of your fund raisers. If you had 3 people in each group there would be enough work for each one to have a hands-on experience. Group interaction might also help solve that problem where one or 2 people want/need the instructors full attention at the expense of the schedule.
Have fun!
Laura

I went to the dollar store for my workshop molds and I'm glad I did.
I told my little group they could also bring molding material of their own as well and 2 of them brought items almost completely unsuitable.
Although with a healthy application of Tuck Tape I was able to cover over the undercuts on one mold and my student got a wonderful pumpkin shaped bowl off of it.

But even that turned out to be a great lesson about the perils of undercuts when casting tufa or concrete.

One of my students has already zoomed past me in the talent department and is making sculpted stuff I'll never be able to do in this lifetime.
Sigh.:)
It must be nice to have art skills.

Just a follow up to say thanks-no big THANKS to all who contributed their know how in this thread.
Our workshop went off with out a hitch. We got lucky and had a good day for it, so the mixing crew outside was able to survive in shirt sleeves. Once mixed up it was ferried inside in 5 gallon buckets and distributed. In all we had more than 20 people take this work shop, with lots of folks coming out to watch too, so it was very popular and well recieved meeting topic.
Maybe the only draw back-it went so well, we'll have to do it again!
Thanks once more to every one who offered advice.
I will try to post some pics of finished projects.

Mark, Glad to hear that you had a great day. It's always better if you can mix outside.
The weather has been so warm in the East. With all the warm days that we've had, I've either been cleaning up Winter damage to the gardens or mixing hypertufa outdoors.
Hope you can post pics too. Billie

Hey woman! How are you doing? :)