help! advice on concrete molds please

[tania]

Hi there, I have made a concrete mold of a slab relief sculpture which i originally carved in clay. Then, using oil (canola) as a release agent I have poured a few concrete positives from that mold. The reproduction is good, details faithful and I am pleased but for the residue that the oil leaves on the concrete. Is there another release agent that is better, ie that would leave a clean, unblemished finish? I then want to coat it in fabriglaze or some such product. Will this waterproof it? Also, I have been using a straight cement mix but is it better to mix with sand and if so, in what proportion? I am so close to getting a good finished product but this last little hurdle is starting to frustrate me so I would be happy to have some advice. Thanks everyone! :confused:

[Landseer]

The use of concrete for a mold is not the best idea, but since it's done...

Im not sure cooking oil is a good idea to use- it's a food, it goes rancid, it leaves residue, possibly paste wax or dish soap brushed on lightly and allowed to dry might be better.

NEVER use straight cement! always add sand at least to it, straight cement shrinks, is very weak and poor, I use a 1:2 ratio- 1 part of cement to 2 parts of fine sand by volume. You can go 1:3 if you wish too- both ratios are better than pure Portland alone. You also want to use the least amount of water you can to get the materials moist and workable, and cure it wrapped/sealed in plastic for 7 days.

[tania]

thank you so much for the advice. I am going to try it out right now and will let you know the results. Tania

[anatomist1]

Dried dish soap sounds good to me. I'd like to hear about how it works. I may want to mold some gargoyles some day.

[tania]

to landseer who replied to may last query. thank you for your help. I realized i was knocking my head against a brick wall so I decided to stop messing around and I poured a silicone mold. this worked out perfectly and i have pulled many copies in the last couple of months. i am delilghted with the results. Without the helpful hints and advice from you and others here at this site i may have given up. Anyway, i am now selling the casts locally now and have had excellent reaction to them. So there you go, success!!! thanks again, tania

[tania]

hi anatomist1, well i learned a thing or two on this long long road... yes dry dish soap does work as a release for concrete molds. better yet you can buy a silicone release agent that comes in spray form from Industrial Plastics stores. But.... i seriously recommend NOT using concrete molds because silicone is the way to go. It costs a bit but it is so perfect for the job. I used Silicone 3110 made by Dow Corning. Best of luck, tania

[gardenwhimsies]

As a note for anyone (else) looking for info on release from concrete. As for soaps, try the Murphy's Oil Soap (you kind of get both oil and soap but without the food quality issue). Put it on till it covers all cement and wipe off the excess (expecially in grooves, etc.).

[Mold Man Mike]

Armor All works as a concrete release agent as well. It contains silicone. I always rub the mold surface with concrete to pick up all of the detail before I pour the rest of the concrete in.