Slab work- some tips learned over time

[Mark C.]

There has been many a thread on cracking and loss of slab works lately .So I'll share some tips as loss rate here is extremely low. The glaze is the only part that can mess up the work most of the time.

Yesterday I unloaded a few glaze fires of Porcelain. In those loads I mix in some slab work-they are press molded into a form.My thrown work is mostly Dave's Porcelain and a little Babu or 50/50 Porcelain 

I could make this form from clay and cast it in plaster and press into that but I'm lazy. So when at thrift shops while traveling at longer/futher away shows I head to thrift shops looking for forms to do slab work in.

I use a special body for this work if the forms are large that handles this well. Its called 1/2 & 1/2 its Daves porcelain and WSO- about 50/50 .That's a cone 10 body but any body made for hand building will do. You need some tooth in it so it can handle this stress.

On the smaller forms I use more Dave's and some scrap with that 1/2 and 1/2 mixed in.I adjust the mix in my peter pugger depending on the forms-you can wedge this as well. For scale that large platter is 21 inches long-its big boy. The others are not small either except the small long oval.

We get very little loss from the e forms unless they start out to dry (the clay) or to wet. They are trimmed with a needle tool around the form  roughly and then edged with a small tool called a round edger from Chineseclayart. It tracks along the plastic form edge and gives  an nice round over.We wet a sponge and hit this edge when done.

here they are

https://www.chineseclayart.com/Store/Tool?page=4

I throw about 99% of round forms so ovals appeal to me as they are not round and just about all my forms are round-so I keep and eye out for interesting ovals Most are plastic forms. They are in most thrift shops for next to nothing. The curved footed forms in photo are some new Japanese forms I found last summer at a show-super well made in Japan-maybe  a dollar or less.A local Sushi place bought my 1st ones last fall and used them in a commercial. Food pops on that black glaze.

First forms are wiped with Pure lube from Laguna-

https://www.axner.com/pure-lube.aspx

folks seem to like to use anything BUT this which is made to release clay . I do not use motor oil -hair grease or bearing grease of murpys oil soap or lard , molasses or a long list of things bought everywhere but a ceramic place. So if you have issues with release it could be the super glue you are using  to release the clay from the form .-nuff said on that 

Slabs are rolled out on a Bailey electric 30 inch roller and allowed to stiffen a bit if to soft Cut to oversized then draped into form and then fingered into the footed areas (the deeper areas of form). Cut with a needle tool around form and allowed to stiffen a bit more than edged with special tool. After a bit of dry time run a fingernail or a similar tool just under the top edge

Since these are all inside molds the form shrinks away nicely. I often sun dry them and flip them out when stiff enough to support themselves. If they have texture on backs I will knock that down with metal edge and sign and sponge. No slow drying around here at that point heat them in winter or cook them in sun in summer. I stack them in bisque and do not baby them at all. Cracking is a non -issue

The trick is the right clay body and the right handling of them during the making.  In terms of the glazing I always just brush glaze on the backs as they like to run as does all porcelain with shiny glazes. Brushing puts the least amount of glaze vs dipping to pouring on the backs. The fronts are brushed with another glaze (I call underglaze but its not what most think of under glaze at lower temps)It just my usual glazes applied under another glaze of same types. Then the outer glaze is poured over. This is just on the fronts-I like the long spouted funnel pitchers I have spoken about before as they give you precise control on where you are pouring .They come in two sizes and I use them both.

I shot the backs so you can see how they are glazed. The footed forms are poured on the backs as they have feet and can take a thicker glaze without running.The feet are thrown rings cut and applied when firmer. These two clays (the slab and the ring) need to be about the same posture content or you will get cracks. The other point is since this form is curved it has to be a bit thicker or it slumps badly as does most porcelain so there is a learning curve with this form if you attempt it.

These pots are bound for a show somewhere some time ,just at this time no-one knows where and when. I think its time to make a glaze called Covid-19 to reflect on this time for me.

Mark Cortright

 

[Babs]

Yes I have collected the plastic, glass wooden forms for ever.

As I sometimes struggle tp get consecutive days for clay and as I find my body needs breaks in  throwing sesions I make slab built forms..apart from releasing the edge with my finger nail or tool I don't baby them either.

Out in the sun till firm enough as above then on racks to dry.

Sometimes I use resist stencils with slip on red clay.

Sometimes just impressed objects and additions onto form.

 

[blackthorn]

Sound info and useful. Thanks Mark and Babs.

[oldlady]

thanks, mark.   your explanation of the kind of work it takes to make a good slab clay is enlightening.    makes me realize that i probably will never find a great clay like the one i have been using for years.   i know you love the joke that is it's name, little loafers.   it really is perfect for both throwing and slab work.   i am reluctant to use the last 25 pounds that i have, knowing that the new stuff just is not the same.   

it is fine enough to repeat the tiny lines, human hair size, that are in gingko leaves and strong enough to stay flat on the wide rim and bottom of a very large platter.   i guess i will just have to buy some of it and open the bag for over a year until it becomes almost useful.  i have been spoiled by it's  perfection in the past.

 

[Mark C.]

Yep my feet are way to big for little loafers-you have a good memory.

[Rae Reich]

@Mark C., good tip about brushing on the backs! Thanks