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Slip Recommendations?


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I am using low fire Clay usually Nasco Red or White Earthenware Clay for slab building.  When i'm initially done constructing the items they look great.  However after usually 2 days of air drying I start to see small cracks in many of the joints.  So far I tried slip from the base clay with only water added, pure water as a slip, and viniger as a slip, from recommendations I have gotten from various individuals.  The other recommendation is making paper slip which I have not tried yet.  And yes I am scoring the surfaces to be joined together as well as reinforcing them from the inside. 

Right now I'm looking for some ideas or a conscientious of ideas on how to improve the joints.  Which of the above is the best in your mind or is there something even better that works for you?  Some of my second thoughts on the mater is perhaps I'm using the wrong clay to start with for Slab building?  Perhaps a clay with grog in it that has a lower shrinkage rate might be a better fix?  

Please not I''m a newbie at slab building as all my prior experience was on a wheel or pinch potting?

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It might be that your clay wants to go a little slower. Are you covering the pieces for a couple of hours to let the joins "marry" properly? Cheap and easy to try..

 

It's been my observation that attachment slip comes in many flavours and thicknesses, everyone has their favourite, and they all work with the right technique.

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I second both previous posts.

 

I have found that for slab building larger items, I use one of two options. 1. I have a really gritty but strong white earthenware that practically stands up and walks out the door (WS-5 Laguna). 2. If I want to use a smoother, finer clay I get everything cut out then let it get pretty stiff before I start joining the sides. Then the clay does not sag or pull on the joints, it stands better on its' own.

 

The thin coil Neil mentioned, in the corner seams, works great with my tissue box covers which are about 6 1/2" tall. And I agree with Diesel Clay about the slow drying. I take it a little farther ... I put all my items into big plastic bins with lids for about a week after I make them. It's not 100% airtight so it does dry but very slow. I check on them every day and if I see a tiny crack form I will put some slip in it, maybe spray it with water, then smooth with a rubber rib. So far I've had very good results.  

 

The slow drying also helps prevent warping, which is always a good thing. It also gives me a little more time to deal with the pieces, which is awesome because I have a small child and the longest I'm ever able to stay in the studio at one stretch is 2 hours.

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yep, slow drying usually avoids cracks for me - i leave it covered in plastic for a week or more (depending on size of piece) before air drying.  supporting the piece well helps too if the arrangement is such that gravity would tend to pull the slabs apart.   gl.   rakuku

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Thanks for the tips.  Considering the design of my pieces I think the biggest help will be allowing the slabs to stiffen and shrink more before joining them.  To give you a basic idea of shape I'm working with think about 3 Square Vases that are joint together and the center vase is upside down.. 

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Make sure all the parts are at the same moisture level when attaching, and dry them slower. It doesn't hurt to run a very small coil inside the corner seams.

 

Neil's point is the key. The most common reason that pieces joined with slip crack apart is because the moisture levels in each piece are different as they begin the drying process.  Since most of the shrinkage that green clay experiences happens in the early part of the drying process, if two joined pieces start out with different enough moisture levels, one of them will inevitably be shrinking faster than the other, and, presto - drying cracks occur.  This same thing can happen when crack-producing stresses build up in a thrown pot because one side of the pot is exposed to a draught and the other side is not.  About now the porcelain-using folks are nodding because this shrinking problem is probably at its worst with porcelain bodies (nothing like throwing a big platter, setting it on the table and coming back an hour later to find a big crack has suddenly appeared in the middle - this is why porcelain users advocate VERY slow drying)

 

Yes sometimes joins don't crack, and yes sometimes a bit more slip will fix them if they only crack a little.  However, the best plan of attack is to equalize the moisture content of the two pieces BEFORE joining them, by putting them in a damp box for a day or two before AND after you join them.  The damp box is very humid, and basically forces the moisture content of everything inside it to come up to the same level.  So when individual pieces are eventually taken out of the damp box, they are all beginning the drying/shrinking journey from the same starting point.  Putting them back in the damp box AFTER you join them ensures that any incidental unequal drying that took place while joining is re-qualized. 

 

A damp box is basically any container that can be sealed, and that you can put a piece of plaster of paris in.  Plaster is like a sponge.  Although we normally use plaster bats to dewater wet clay and slip, we can also use them in the opposite direction - that is, to put water back into clay! :lol: .  I have used old paint cans, plastic bags, big buckets, or any manner of container that can be sealed and withstand a damp environment.   That said, the best container I have seen are those big plastic bins with covers you can get at most hardware or home good stores.  Just mix up some wet plaster and pour about two inches or so evenly all over the bottom of the plastic bin.  Jiggle or tap gently to get a very smooth surface on the plaster.  When it is dry (a few days later) just pour in enough water until the plastic is saturated, but not so much that it pooling on top of the plaster.  I get cheap humidity meters off EBay for a couple of bucks and keep one in each bin to monitor humidity levels. 

 

One last thing.  If you prepare your clay pieces in this way, you can dispense with the whole fork/toothbrush scoring ritual for joining that every ceramics student is taught as gospel.  IF moisture content is equalized amongst your pieces, just use some slip of the same clay which is the consistency of chocolate mousse (the consistency is VERY important).  Use it like paste when you were a kid.  Glop it on and smooth it off with your finger, a wooden tool, or whatever smoothing process you want.  I have been doing this for a long time and cracking is no longer an issue I worry about.  I know this last bit will be absolute heresy for some, but don't believe me, just try it yourself!  :D     

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Curt

 

I think that slow drying process might be a real important part to where I'm having issues.  Some of my first pieces that had the worst cracking were built in the winter months with the humidity outside a under 10%.   Lately it is warming up and the humidity is rising to the 60-75% ranger and i'm not seeing as much cracking and when i do see the cracks now they are basically hair line cracks.   While I making changes all the while trying to eliminate the cracking any improvement I probably saw was just random compared to the increased humidity.   

 

My only concern is warp age or sagging if the pieces are too wet.   Basically  the center ot my pieces  have a top slab ranging up to 8" X 8".   I had tried supporting this with wetter clay in the past and found that the shrinkage of the side walls lowered the roof resulting in a humped rather than flat roof.  I think I willhave to search for something that can support that roof yet  is flexible to the shrinkage.

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Hi Dennis,

 

What you are saying about humidity makes sense. Greater humidity in the air around your pots is working to rehydrate all the parts of your pieces. At the same time, they are drying and shrinking due to air flow around them sucking water out. The only question is which process wins the race. If they are drying faster than than the humidity in each part can be equalised, then cracks will occur. Again, with a damp box you can shut down the drying until the humidity has been re-equalised.

 

If humidity has been equalised throughout the piece, you will find that you can probably dry very quickly, in hours, not days! The only rule is that all parts of the piece MUST dry at equal speed (otherwise drying stresses develop). A convection oven with your piece on a spinning turntable inside it would be ideal for speed drying (although probably impractical for most of us).

 

Regarding your roof pieces, is it not possible to add them later when the clay has become a bit stiffer?

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Hi Dennis,

 

 

 

Regarding your roof pieces, is it not possible to add them later when the clay has become a bit stiffer?

 

Yes it would be possible however we then get into the issue of matching the dampness of the body with the roof?  If it gets too stiff for both pieces then I fear cracking at the joint again since the slip will be much damper than the rest of the body.    But this might be something for me to experiment with using your sealed containers.  

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I understand the concern.  What I would suggest is put both the roof and the rest of the piece in the damp box.  How stiff they become (very or not at all) depends on the actual humidity in the damp box, and this is why I like to monitor the actual humidity levels.  If you find after a couple of days that the pieces (although now of equal moisture) are still too soft, you need to reduce the humidity of the damp box.  Just empty the damp box out and leave the lid off for a day, so that some of the moisture in the plaster bat evaporates.  Then recover the damp box and put your stuff in again.  It might take a few trys, but eventually you will get the right humidity level and some sense of how to maintain it.   Again, a cheap electronic humidity meter from EBay will save you having to guess what the humidity level is. 

 

If moisture in both pieces is the same, AND your slip is the consistency of chocolate mousse as advised above, then you should be good to go.   I see many people slopping on water and soupy slip to join pieces and you can understand based on all this why that consistency is not right.  The slip should be as firm as you can get it, but just soft enough to spread and work without trapping air bubbles.  Hence the chocolate mousse benchmark. 

 

Again, give it a try and I think you will be pleasantly surprised.

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My only concern is warp age or sagging if the pieces are too wet.   Basically  the center ot my pieces  have a top slab ranging up to 8" X 8".   I had tried supporting this with wetter clay in the past and found that the shrinkage of the side walls lowered the roof resulting in a humped rather than flat roof.  I think I willhave to search for something that can support that roof yet  is flexible to the shrinkage.

 

In addition to everything else, maybe experiment with slightly thicker slabs to prevent the sagging. The first couple of tissue boxes I made were thinner, I think 1/4" slab rolled and in a low fire grog free clay.  After that, I started using more like 3/8" slabs AND the WS-5 with grog. The tops are about 6" square, so not as big as your forms, but it still might help. I also start them out drying upside down so the top stays flatter.

 

If you look at all the photos, you can really see the warping with the thinner slabs of low fire clay: https://www.etsy.com/transaction/200353319

 

These are with slightly thicker slabs and the groggy clay: https://www.etsy.com/transaction/243604594 and https://www.etsy.com/transaction/260697855

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  • 2 weeks later...

Well I hot my pieces back fom my first glaze firing and my second bisque firing..

From the bisque firing I only had one crack out of 8 pieces.  The slip i used for that firing was    a 50% viniger, and 50% RO Water mixed with the clay to a very soft consistency.  Do to the warmer humidity in the basement it took a full week for the clay to harden.  The batch I'm working on now is using the same slip recipie but I'm wrapping the pieces in plastic for a free days before alowing them air dry.

The glazing pieces I had done were my first glaze in 20 years and I have very mixed feeling about them.  These pieces were textured and I made groves in  patterns to signify a brick or stone surface.  For glazing I used Duncan underglaze in yellow followed with a layer of AMOCO F glaze that a Rubbed off the high spots.  after that I added three coats with a mixture of AMOCO Clear and Chocolate Brown F Glazes.  The idea was to decrease the brightness of the yellow.  

The way the black reacted on the pieces varied considerably.  For some of the pieces it stayed in the low areas faming the bricks beautifuly.  For others it seemed to spread out from those areas almost looking like I put a transparent coat of black on the pieces with only slightly increased thickness in the groves.  

My next batch that I have grazed but still needs to be fired I skipped the yellow under glaze. and just used the black in the Glaze in the textured parts, then whent with one coat of clear, one coat of chocolate and a final coat of clear. 

On the next trip to my supplier I'm thinking about getting AMOCO black velvet under glaze for the areas i want black like the lines between the bricks and stones. 

Unfortunately all my pieces are in use now in my Aquariums otherwise I'd post some pictures.   

A quick question though on underglazes, When I only want to undeglaze the low points is it possible to simplu apply the underglaze everywhere than either gently scrape it off or sand it off the green ware in the high atras?

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Unfortunately all my pieces are in use now in my Aquariums otherwise I'd post some pictures.   

 

A quick question though on underglazes, When I only want to undeglaze the low points is it possible to simplu apply the underglaze everywhere than either gently scrape it off or sand it off the green ware in the high atras?

 

Yes, applying underglaze and then wiping off the high areas works . . . you can do this at either the greenware or bisque fired stages.  If doing at the greenware stage, I'd suggest using a flexible rib to remove the excess underglaze as a sponge/cloth/sand paper/scrubbie  may also wipe off the top layer of fine clay particles, leaving a rough surface on the high areas.  At greenware, medium or cheddar cheese leatherhard will work better than totally dry. 

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