O.K., here's one.
I want to make bas-reliefs about 3 to 4 ft. high. I will use clay (Duncan's powdered clay)....but I was wondering how to make them stronger so that they would last longer when hung outside and exposed to the weather.
What would happen if I mixed a little smooth concrete (liquid, of course) with the clay (when the clay was ready for shaping/sculpting) and then fired it?
As you can see, my knowledge of chemistry is quite lacking. Lina.

Lina, I don't think that approach would work for you. The clay and concrete would mix but when fired the cement would not hold up and crumble away. Have you've seen another post in this forum about moisture in portland and how it ruins it? Well that portland could be revitalized if it were fired above 1200 degrees and reground to it's original flour consistancy. What is done is all the moisture is driven out at a molecular level. Which is part of the process when portland is made. So that's what would happen when you mix your clay and cement. Firing is good for the clay, bad for the cement. Hope this wasn't too much info.

Maybe firing your clay then layering cement over would work to make them stronger for outdoor purposes.

I'm not familiar with Duncan's powdered clay. Is it a low-fire or high-fire clay? A high-fire clay should be impervious to weather. A low-fire clay like terra cotta (etc) might not be weatherproof unless it was kept dry. Maybe you could talk to the people at the place where you will have it fired and ask for suggestions.

Concrete can't be fired without it becoming porous and brittle.

Sue

Sue is correct talking with the place where you get your clay will be a better chance to get it to last outside.

Concrete should not be fired, the concrete, and most cement mises will explode. If you decide to try this you may risk everything else in the kiln, and a costly replacement for the shelves and liner bricks, not to mention the coils that are outside of the liner bricks.
For mortar and brick that needs to withstand fire there is a fire clay, dry that can be added to mortar mixes to increase the ability of a firplace, fire pit, bbq pit etc to withstand fire, especially fire where the heat is not as consistent. Usually the mortar needs to be redone or the firebrick replaced because it does crumble over time.

I did try using raku glazes (dry oxides, and damp glaze)and then torching it with a propane torch, oh did it ever crackle!!!!Did not pursue it any further. The guys at the materials place looked stunned when I told them, I guess I am lucky.
I also asked if it was possible to use fireclay as an additive so that I could kiln fire. They recommended against it.
And there are no stupid questions or questions that can not be asked more than once.
Dena

"...The guys at the materials place looked stunned when I told them..."

Don't you just love that look? I used to get it all the time at my local hardware store/lumber yard. Then they just tensed up a bit when they saw me coming, preparing themselves. Now, I mainly talk to Lee, who leans forward and asks "What are you doing now?"

Thinking outside the box sure rattles some people, doesn't it?

BTW, Dena, I was going to try that myself, so thanks for the tip-off!

Sue

Laa_Laa... Have you considered using hypertufa for your bais relief? I have been thinking about that. dosent need to be fired, will stand up to weather. you can lay your 'tufa on a bat, covered with plastic.Shape your big areas,mist and cover, and let it sit a while, then carve in some details,mist and cover, let sit awhile, and carve in more details. If you keep your work covered and misted it can be workable for two or three days.I started carving tufa after reading "garden of mu's" instructions on making grots,as seen in this forum. His tutorials were easy to follow, and the results were terrific. Marley and Denna also do some carving. Great for original work, not good for ceramic molds.

Laa_Laa , It isn't my idea, I've seen it from a very well known artist who makes huge reliefs, even 12x5 ft ( in sections). He makes holes for screws, well hidden in the design, after finishing everything he secures it on MARINE GRADE PLYWOOD. Let us know how you succeeded.

Andrea

laa-laa, have you tried your bais relief yet? I made and sold a wall hanging, 'the old man of the forest' using 'tufa. A hanger was embedded in the back, and chicken wire was used between layers for strength. I am now contemplating a larger relief showing horses. Sure would like to know your experiences if you tried it.
kobold, is there any special reason why the 'tufa was applied to marine grade plywood? I plan to use spacers adheared to the back to provide space between the hanging and the wall. these being covered with felt pads for protection of the wall on which it will hang. have you done any of these?

Marine grade plywood glue is a fully WATERPROOF structural adhesive. Not water-resistant, waterproof.

The last thing you need after you finish a detailed panel is for it to start warping.

Sue

Marine plywood is NOT rot proof.

Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.