[Ginny C]

How do you make a plaster bat with a concave surface to hold a batch of thick slop for drying? I think I saw a video showing some oval bats but I don't remember if it told how to make them. Won't the plaster level out while settling, "seeking its own level?" Can I just pour the plaster into some large plastic container of the needed shape and size? But, again, how do I keep the surface from going flat? Would I need to put vaseline on the inside of the form so the plaster will release?

Also, for just drying, not wedging, can it be thinner and therefore lighter so it would be more easily moved out of the way when not in use? It's for my home cellar studio. I've been drying clay on the heavy plaster bat my husband made for me for wedging, which is so heavy it has to stay put. And it was not potter's plaster and I plan to ditch it (Yikes! Where?), as I now know I can wedge on a wood surface. I want a plaster surface just for drying clay, concave so it will hold more sloppy clay and thinner so I can move it!

Thanks!
Ginny C

[Diane Puckett]

I know people who use this system for making plaster forms.
PSPottery.com

There is good info here Plaster and Molds

I thought there was a way to pour the plaster into a rectangular plastic box and, at some point, using a large, glass bowl to form the indentation. Then you can just leave the plaster form in the box for storage. I am curious to see responses about thickness, as size and weight seem to be major issues in plaster forms.

[neilestrick]

If you've got a concrete floor, just use that instead of the plaster. Put down an old pillow case and lay the glop on it. Several of my students do that in their utility rooms at home since we don't have the space here at my shop. Works great!

If you do a plaster bat for reclaim, it needs to be THICK, so absorb as much water as possible. A thin slab will saturate too quickly and make the clay drying process much slower. There's no reason you can't use a flat slab.

[Lucille Oka]

You will need several things to make a concave plaster bat. First get actual #1 Pottery Plaster from your ceramic supplier and you will need a prepared cottle box of the size bat that you want. Make a smooth and even clay mound. You can throw this on your potter's wheel, or make it oval. DO NOT try casting this on your wheel head. Be sure to allow for several inches above and around the mound when choosing your cottle. The cottle will hold the model and the plaster. Place the clay mound in the center of the prepared cottle. Correctly mix and pour your plaster down the side of the cottle until it fills up; allow for at least 2" or more on all sides of the mound. After the plaster has set and is cooled, invert the mold and remove the clay. Clean up the corners and edges of the bat, prop the mold on its side for drying.

Also, if you have any slump molds they can serve double duty as a concave bat. I use the flat backs of slump molds to wedge up small amounts of clay from time to time. I also use the concave side to reclaim slip. Don't let the clay dry in or on the mold.

These instructions take into consideration that there has been previous plaster working knowledge and experience.

[TJR]

Ginny;
I just came back from the clay supply store with a 50# bag of #1 Pottery Plaster. It is possilbe to cast a plaster bat with a depression in the centre without using cottles.
You will need dry cleaner plastic, mold soap or dish detergent, waste clay that you will not use again, and large plastic tub. Also, plaster.
1.You will be working upside down. You will be right side up, but your form will be upside dowm.
1. Again-Make a solid clay shape out of leather hard scrap clay the size of the depression you want. Probably rectangular.
2. Line you plastic tub with thin dry cleaner bag. Tape the edges to the sies so they don't flop around.
3. Place your leather hard shape in the centre of the bottom.
4.Cover the clay with mold soap, avoiding bubbles.
5.Fine a bucket or basin that you don't mind throwing away.
6. Fill about 2/3 up with the coldest water you can stand.
7.Slowly layer plaster in until you have and island sticking out the top.
8.Mix this in from underneath. Gloves[rubber]would be a good idea before touching the plaster.
9.When you can drag your finger across the top and it leaves a mark, you plaster is setting.
10.Pour a thin layer covering your clay shape and the bottom of the tub.
11.At this point you could add celulose insulation to your mix to lighten your mold. Use a dust mask. Don't dilly-dally, keep pouring.
12.Yuor plaster will heat up as it is setting. When it is cool again you can remove your mold.
13.Flip your plaster, dig out the clay with a trimming tool, and there is your mold.
14. Discard the clay, wash all your tools, and DO NOT pour excess plaster down any drain.
TJR.

[Diane Puckett]

Another caveat a friend and I learned the hard way. If you are using the same bucket or trash can again to mix more plaster, it must be completely clean of all residue before reusing it. We needed more plaster and just mixed more in the same bucket where it set up before we could even pour it into our mold. Things turned out okay in the end, and we laugh about it now, but it was not so funny at the time.

[TJR]

Diane Puckett, on 05 January 2013 - 05:20 PM, said:

Another caveat a friend and I learned the hard way. If you are using the same bucket or trash can again to mix more plaster, it must be completely clean of all residue before reusing it. We needed more plaster and just mixed more in the same bucket where it set up before we could even pour it into our mold. Things turned out okay in the end, and we laugh about it now, but it was not so funny at the time.

Diane;
Plaster is no laughing matter! I always throw my buckets away and BURN MY CLOTHING! Just kidding. :angry:src="http://ceramicartsdaily.org/community/public/style_emoticons/default/angry.gif">
TJR
Why don't these emoticoms work?Frowning guy third from the top on the left.

[Marcia Selsor]

For making large concave plaster bowl shapes I have made casts from large metal "sausage mixing" bowls 24-30" in diameter. They have round bottoms for fitting on machines.
I have also used glass forms coating them with vaseline for mold release. Plexiglass is the cleanest thing to cast over.If the plaster doesn't want to release, heat it up with
hot lamps. it will come loose.
I have an article in PMI on making oval convex molds. “Over the Hump”, Pottery Making Illustrated, May/June 2008 pp 31-34.
These could in turn be used to cast convex shapes.

Marcia

[Lucille Oka]

Flexible rubber plaster mixing bowls are a very good investment if you will be doing a lot of plaster casting. They last for years and years. They come in different sizes and can be quickly cleaned with a piece of old crumbled newspaper or damp paper towels.

[Ginny C]

Marcia Selsor, on 05 January 2013 - 07:06 PM, said:

For making large concave plaster bowl shapes I have made casts from large metal "sausage mixing" bowls 24-30" in diameter. They have round bottoms for fitting on machines.
I have also used glass forms coating them with vaseline for mold release. Plexiglass is the cleanest thing to cast over.If the plaster doesn't want to release, heat it up with
hot lamps. it will come loose.
I have an article in PMI on making oval convex molds. “Over the Hump”, Pottery Making Illustrated, May/June 2008 pp 31-34.
These could in turn be used to cast convex shapes.

Marcia


It looks like PMI back issues stop at the Sept-Oct 2008 issue! Is there another way to access your article? Hmm. Maybe our local library? I'll try that.
Lots of good answers already to my post, so I think once I get out of this boot (bunion surgery 2 days ago), I'll be able to make what I need. Thanks, everyone!
Ginny

[Lucille Oka]

Ginny, if you are going to the library, read books on mold making for ceramics; this will help you a great deal. It will show you tried and true techniques for what you want to do.

If you are planning to do more plaster work find out about plaster, what types to use, how to use it, and the proper amounts to use and the best way to mix and cure. Go to the US Gypsum website and look under 'plaster for ceramics'.

Get your plaster from your ceramics supplier #1 pottery plaster is very good for a wedging surface. If you are making plaster molds for casting, Hydrocal White is very durable and wonderful to use.

But first and foremost get those books on mold making, you won't regret it.

This post has been edited by Lucille Oka: 11 January 2013 - 04:33 PM

[perkolator]

Diane Puckett, on 05 January 2013 - 03:20 PM, said:

Another caveat a friend and I learned the hard way. If you are using the same bucket or trash can again to mix more plaster, it must be completely clean of all residue before reusing it. We needed more plaster and just mixed more in the same bucket where it set up before we could even pour it into our mold. Things turned out okay in the end, and we laugh about it now, but it was not so funny at the time.

it's not ESSENTIAL to remove all the plaster residuals from the bucket, and is always easier to clean the bucket once the plaster has finished setting up (it simply just breaks out when you slam the bucket). Besides WARM WATER and AGITATION, already CURED PLASTER is a plaster accelerant, so keep that in mind.

for those who need to do a lot of plaster mixing, get yourself a rubber animal feed bucket - works WONDERS for plaster since you just flex/step on the bucket to break up the plaster for cleaning.

[Marcia Selsor]

Don Frith's book on moldmaking is a classic.

I use my large concave forms for forming large bowls for fountains as well as for drying out slip. Depending on how much slip you need to dry out, flexible feed buckets would make a good form for casting a slip drier for about 2-3 gallons of slip. They also make great glaze buckets for dipping platters and plates.

Marcia

[Wahine]

I don't have a local supply for pottery plaster. Someone recommended plaster from a building company. I believe it is the type used to repair old plaster walls. Has anyone used this? I only need something for recycling clay and batts. I appreciate any replies.

-chantay

[Lucille Oka]

Clay suppliers usually offer pottery plasters also, if they don't, ask them to try and get it for you. 50 pound bags of Pottery Plasters are easier to carry, the name and kind of plaster is printed on the bags and they have air hole punctures on it. Store the bags in heavy closed plastic bags, up off of the floor. And remember to keep the bags tightly closed between usage.

The other plasters such as those you can buy at building suppliers are different. They are softer and you will find that the surface of those mixed plasters will lift off on to your clay and ruin your clay. They are not meant to be worked on they are just to be applied on to a wall and then painted.

Do yourself a favor and do what you can to get plaster for pottery/ceramics such as #1 pottery plaster, Hydrocal White, Hydro Stone, and etc. If you have to get it from another city or state it is worth it.

Be sure to mix the pottery plaster properly. Go to US Gypsum website look at (plaster for ceramics) and use their calculator to determine how much water and how much plaster you will need for your particular task.

If you can get a new copy of 'Mold Making for Ceramics' by Donald E. Frith there is a 'Volume and Mix Guide' in an envelope on the inside front cover. If you get the book used, be sure this is included.