Jump to content
douglas

Plaster Vs. Bisque Bats And Molds

Recommended Posts

The studio I learned in had a strict no plaster policy. The risks of plaster getting into clay and causing explosions or pop-offs after firing were drilled into all of us. Therefore when I use molds, I make them out of clay and bisque them. I use plaster to reclaim clay but never use scrapers on the plaster, just wedge clay and clean off occasionally.

 

I see lots of respected potters using plaster bats or molds to throw plates, the seem to use ribs or trim tools to cut off excess clay and don't seem too worried about a stray speck of porcelain being scraped into the clay body.

 

So I am suspecting I am overly cautious with using plaster.

 

Can anyone share their experience of using plaster to throw with -- what precautions to take. Do you throw away excess clay in case there are porcelain particles,  or do you reclaim it? What is the right level of care to take when working. Is there a shelf life to bats, where you need to stop using them or they will start to degrade and deposit plaster into your clay?

Share this post


Link to post
Share on other sites

I can see not using them in a teaching studio but I’ve had zero problems using plaster batts for many years in my workshop. For cleaning them off I use a yellow Sherrill rib. 

 

Haven’t had to replace any of mine, think clay composition and fluxes used in it will make a difference to the amount of salt fuzzies you get on them. Plus making the plaster up with the correct amount of water so they are strong. I use wood dish racks to store and dry  them out between uses. I have had no problems recycling porcelain trimmings etc. I probably shouldn’t but I use a metal turning tool to undercut and trim away excess clay after throwing and I still haven’t had any problems. 

 

I made mine using No1 pottery plaster and springform cake forms, without the bottom piece, on a piece of plexiglas which made a very smooth surface. Ring of clay around the outside. Fast to make and way less expensive than buying them. I have 4 sizes from about 8†up to 16†Embroidery hoops or cutting the top of a 5 gallon bucket works too or use cake tins. 

 

 

Not having to wire off platters is a huge plus for using plaster. What works brilliantly for me is using one of these Ziem Bat Mates on the wheel head. The plaster batts stick to it really securely, even the 16†ones don’t budge. I don’t use it like they sell it for, with plaster you don’t need pins, just the Bat Mate on the wheel head.

douglas and S. Dean like this

Share this post


Link to post
Share on other sites

I use homemade plaster bats in my production-10 tons a year of porcelain for many many decades with ZERO issues from plaster. I did drop one on my foot once and it hurt.

I would not be able to make the production I do without plaster bats- they dry the bottoms out evenly and fast. The only drawback is they are a bit heavier to move on ware boards.

The whole explosion plaster phobia is like many wives tales you hear in ceramics-like cones going bad as they are to old or air bubbles blowing up pots or glaze getting old or bad gas from the supplier. Never seen it happen in 44 years.

Over about 15 years I had worn down some (like 40 or 50) of my smaller mug bats . They had a uneven surface so I took my wheel outside glued a 40 -60 grit rough balck sandpaper to a 12 inch wood bat and ground down all the uneven surfaces on the spinning wheel. That fixed them all perfectly flat again. They will outlast me and at least 30 -40 years of production work. If you make them as Min says they will never degrade.I have some from the 70s still in service.If you swap out clay bodes you can just wash them and dry them and bingo they are ready for whatever color clay you now work with.

If you snap one just use waterproof wood glue and glue it up with a clamp-they will last another 20 years glued up as I have a few that I have done over that decades with glue.

I throw a clay pad on the wheel head and stick them to that and use a tool to pop off the bats.

Homemade Plaster bats are the best kept secret of production potters.Just like homemade music is for musicians.

douglas and Joy pots like this

Share this post


Link to post
Share on other sites

I agree with Min that plaster bats may not be the better choice for a community or classroom setting. I had them in my classroom in Brownsville where an equal number of shelves held plaster bats as were available for student work.

I converted the wheels for bats using pins in the wheelheads. Saved significant space.

I agree with Mark that plaster phobia is not justified.

Marcia

Share this post


Link to post
Share on other sites

For these homemade bats - do you drill out for the bat pins?

I just bought a bunch of Plaster to make a new reclaim area and think I might try this. :)  

 

Neither Mark nor I use batt pins but if you wanted to I guess it would be fairly simple to make up a template and drill out holes. I've seen them drilled oversize in plaster and inserting a piece of that flexible plastic aquarium air line tube cut to the depth of the batt so the pins don't wear down the plaster. 

Share this post


Link to post
Share on other sites

RonSa-

100 pounds plaster to 70 pounds water for Pottery Plaster No. 1. I would say yes in order to obtain the strongest with the most absorption for that plaster. If you're using Hydrocal or something else check the US Gypsum website (https://www.usg.com) and search on the specific product(s) you're looking for.

You might also want to check with Mark and others about which plaster they are using.

Regards,

Fred Sweet

RonSa likes this

Share this post


Link to post
Share on other sites

The Lester bat system instructions says 1800 g of plaster and 1200 g of water. This will make a 12 inch diameter x 1 inch thick plaster bat.

I cannot find a link to the bat system. I bought it and made a dozen plaster bats. Works great.

Share this post


Link to post
Share on other sites

Go to the US gypsum site-the measurements are critical as well as your mixing methods.-I'll talk more later on this.

I can post some smaller batches later as its glaze day for me with only a small lunch break which I'm on.No time for this now -two kilns to glaze and load today.

You weigh the water and the plaster for best results-I also have a gallon to plaster ratio. Give me a day or two to post this info or get it from US gypsum

There is no reason to drill the as they stick down fine to any wet clay pad on a wheel head-just use scrap clay and throw out a pad and drop a bat on and its sticks.

Share this post


Link to post
Share on other sites

I use the below ratios.  I work backwards.  Know how much finished volume you need.

 

Also, make sure you have fresh plaster, not old stuff.

 

gallery_59202_704_4968.png

RonSa likes this

Share this post


Link to post
Share on other sites

Here is clay planets guide to making plaster.

I use a jiffy mixer-after mixing -let it sit a few minutes while dropping the bucket to get any air bubbles to rise to surface.

Pour the plaster slowing so air bubbles do not get trapped in plaster.

As to bat making I scrap  the tops across with long metal edge and chamfer the edges to they are not sharp and will not chip.I use a wet sponge when plater is just right to smooth edges as well.

Plaster making is a learning curve and after a few pours you will have it down.

http://shop.clay-planet.com/plaster-2.aspx

RonSa likes this

Share this post


Link to post
Share on other sites

Ron

US Gypsum will send you a nifty calculator that sides -tells you volume -water-and plaster weights-I think you have to write them for this kit

They are very helpful and will send you a packet of all info for mixing Pottery #1 plaster .

Mark

Share this post


Link to post
Share on other sites

I typically use a wonderbat system for my cups and small bowls. The small square foot print saves room on the racks. But for plates and large lids, ect, you can't beat plaster. I use a lot of pure and simples forms. In addition, they give a lot of info on proper use of plaster, mixing, making custom molds. I too was scared away from using plaster from my community studio, but I see why they do this. I strictly use it on a wheel I can clean throughly, but I know I've had work go through with alittle contamination, nothing blew up, glazed it, fired it, would never tell the difference. Still it pays to be vigilant, I use wood tools, and my favorite rib is a red. I've gone on to make my own plate designs because of plaster. I highly recommend visiting the pure and simple pottery web site, they have all the info u need.

Share this post


Link to post
Share on other sites

I am also emboldened to start pouring me some plaster!  Do any of you have suggestions for how to make an empty space in the center of big hump molds?  If I oil the outside of a smaller bowl, can I sink it up to it's rim in the still fluid plaster filling the larger bowl, there by having a lighter mold of the larger bowl?  And using less plaster.

Share this post


Link to post
Share on other sites

I am also emboldened to start pouring me some plaster!  Do any of you have suggestions for how to make an empty space in the center of big hump molds?  If I oil the outside of a smaller bowl, can I sink it up to it's rim in the still fluid plaster filling the larger bowl, there by having a lighter mold of the larger bowl?  And using less plaster.

 

It's hard to push an empty bowl into plaster.  It will want to float.  You need it full of heavyness.  

 

You can use the large flat surface of a hump mould as a drying slab, so all is not wasted.

Share this post


Link to post
Share on other sites

On my 4" thick plaster slabs, I build voids using 2x6 ect, I hold them down by screwing 2x4 over them to keep them from floating up. It can cut up to ten pounds from a fifty pound form. Every little bit counts. I have even created voids with bubble wrap in larger bowl forms, again tacking a stir stick over the form to keep the wrap from floating out of the form.

Share this post


Link to post
Share on other sites
On ‎9‎/‎28‎/‎2017 at 10:12 PM, triolaz said:

Others have said this but to reiterate - use Pottery Plaster #1 not Plaster of Paris

I did use plaster of paris (from the hardware store) for my damp boxes.  I haven't had problems yet.  If I do molds or bats, I will get potters plaster for certain!!

Roberta

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×