Slip Casting Cracks

[tluvs2create]

A friend suggested I try slip casting with the tile molds that I have made, especially a larger more detailed one. I have to admit, I loved the ease of it. However, I am having problems with cracking. What am I doing wrong? I have gone through a whole gallon of the stuff. My first cast was a larger 8x8 detailed tile (with somewhat high relief in the center). This one cracked a lot in the center. The outside areas of course firmed up quickly far quicker than the center, so I left it the dry for sometime. When I returned, it was dry enough to pop out but had a lot of cracks in the center. So, I tried it again and kept my eye on it - still took some time for all parts of the tile to dry enough to pop out of mold and was developing cracks in some areas while other areas were still very wet. So, figure this was too large and detailed a tile to mold. So, I tried a cross shape that I had made, smaller thinner and simpler. However, this one dried quickly in the center and quickly formed cracks again before the outer portions of the cross could even set up. So, I figured that was just not a good design either for casting. Next I tried one of my fairly simple, 4 x 6 low relief tile. This one seemed to dry nicely - no cracks, popped out of mold and then left to dry on drywall board then sandwiched between drywall board. During this drying process it developed cracks in the center and near the edges. These cracks BTW look like the kind you see if you had pudding in your frig. un-covered for too long - not hairline cracks at the edges or anything.

 

So, what am I doing wrong? Is there anything I can/should be doing to prevent this or should I give up on slip casting all together? Thanks much for any help you can give.

 

[Chris Campbell]

Your slip make-up, timing in the mold and drying times are crucial.

It sounds like you are not getting any kind of consistency and all parts are drying at their own unchecked speeds.

In any type of pottery this causes all of the stress problems you are having.

 

I am not an expert, but I can recommend a Lark Ceramics Book ...

"The Essential Guide to Mold Making and Slip Casting" by Andrew Martin.

 

I also looked in the Handmade Tile Book by Frank Giorgini which is my favorite tile book,

but he only cover slips as decoration. HMMM ... I wonder if this is by accident .... : > )

[Lucille Oka]

A friend suggested I try slip casting with the tile molds that I have made, especially a larger more detailed one. I have to admit, I loved the ease of it. However, I am having problems with cracking. What am I doing wrong? I have gone through a whole gallon of the stuff. My first cast was a larger 8x8 detailed tile (with somewhat high relief in the center). This one cracked a lot in the center. The outside areas of course firmed up quickly far quicker than the center, so I left it the dry for sometime. When I returned, it was dry enough to pop out but had a lot of cracks in the center. So, I tried it again and kept my eye on it - still took some time for all parts of the tile to dry enough to pop out of mold and was developing cracks in some areas while other areas were still very wet. So, figure this was too large and detailed a tile to mold. So, I tried a cross shape that I had made, smaller thinner and simpler. However, this one dried quickly in the center and quickly formed cracks again before the outer portions of the cross could even set up. So, I figured that was just not a good design either for casting. Next I tried one of my fairly simple, 4 x 6 low relief tile. This one seemed to dry nicely - no cracks, popped out of mold and then left to dry on drywall board then sandwiched between drywall board. During this drying process it developed cracks in the center and near the edges. These cracks BTW look like the kind you see if you had pudding in your frig. un-covered for too long - not hairline cracks at the edges or anything.

 

So, what am I doing wrong? Is there anything I can/should be doing to prevent this or should I give up on slip casting all together? Thanks much for any help you can give.

 

 

 

Not every mold or every design can be used in slip casting. Looking at the picture of your cast it is hard to tell if you are using a one or two piece mold and if your mold has the proper draft. Looking at the ‘points’ on your mold, which is where your cracks appear. Your cast has no room to move during the water absorption period. It is locked in place. I would change the casting method to press molding and use very soft clay. Always start with a dry mold. Since there is less water to be absorbed into the mold, the form and design are imparted quickly to the casting, making reproduction faster.

 

It is not very labor intensive you just press the clay in using a rubber rib or the back of a spoon and cut off the excess clay. Then turn out in a few minutes. The cast is a little heavier and sturdier. If you are using a two piece mold, leave the cast in the mold a few minutes longer to make handling easier. Score the casts while they are in the molds; be careful not to scrape the mold. Do not leave the casts in the mold too long or the very same structural cracks will appear. Turn out the casts and attach with slip made from the pressing clay, bring the two sides together firmly, clean up the edges.

Just a note casting slip can be made into 'modeling' slip by pouring the liquid out on to a plaster bat then when ready wedging it up. I Hope these suggestions help you.

 

 

[Madison Mitchell]

tluvs has already tried press molding, and was having problems also :

 

http://ceramicartsdaily.org/community/index.php?/topic/602-tile-making-with-mold/page__fromsearch__1

 

From what Lucille has written, I think she is on to something. Since the arms of your cross are drying faster, it kinda looks like the sharp corners are acting like wedges, and giving you cracks.

I am not expert, but if this were my project, I would try to speed up the drying of the center with a heat gun, or blow drier, and get the whole piece to the same dryness, as much as you can. It might not work just right the first time, but after you try it a couple of times, you might get the timing right, and be able to pop the whole thing out; then, if it has consistent moisture throughout, it might dry crack free. Even out the moisture, as Chris suggested.

You'll get this solved, eventually!

[tluvs2create]

Yep, was having trouble with press molding this larger more detailed tile - getting creases. That is why my friend suggested slip pouring. In fact she says she has poured thousands of tiles and never had this problem. THese are just one part open-face tile molds. This large one is an 8x8 square with a relief in the center of it - that is where it took the longest to dry because obviously that is where more slip was needed to fill out that center relief. However, that is where it is cracked the most.

 

The cross dried the quickest at the smallest, thinnest area where the arms all come together. I have yet to press that one, but probably could with little trouble for it is not as detailed as the larger square tile.

 

But, why did the smaller 4x6 less detailed tile look fine and pop out of mold fine, but then start to crack in the center and some around the edges only after a day or so of drying? Do I need to dry it slower - mist it perhaps with some water and cover with plastic for a while? Again, these are not little hairline cracks they are fairly large, wide cracks.

 

One other thought? I placed an order for some glaze on Friday and they would not ship until Monday, they said because it was so cold. Got me thinking, it was cold when the slip shipped (although, it was here the very next day), just wondering if the cold could have affected it in this way?

 

Thanks for your thoughts and suggestions, I really appreciate it.

[AmeriSwede]

First my disclaimer, tluvs. Though I don't profess to be a professional with or know all about slip casting, I do have a few years of experience under my belt, all with commercial molds though. Additionally, I've many many years experience with casting wax, metals and glass, and some problems seem inherent in some of these materials as well. One thing that I've learned through the years is that when one has interior planes intersecting at sharp interior angles, because of shrinkage of the cast materials, this can often place additional stress on the material, resulting in cracks. The cracks normally show as an extension of the planes themselves... pretty much as the cracks in your cross tiles.

 

In this illustration the forces of shrinkage (magenta arrows) show how the clay cross is drying and pulling away from the sides of your mold. Notice that this force can continue at the acute interior angle as the forces of one arm of the cross pull away from the forces on the adjoining arm at the same corner (red dotted circle). If this occurs it would be reasonable to expect this to happen on more than just one corner and most probably on all interior corners, as it seems your photograph illustrates.

 

 

One thing that I've learned over the years to help alleviate this problem is to build up the problem area with a little more of the material that will be shrinking... sort of a reservoir of material reducing the stress levels on the direct corner. This can be done in the design stage to incorporate the addition as part of the overall design or can be designed so that it will be easy to eliminate after the form is removed from the mold.

 

In regards to your cross... if I chose the latter solution, it would involve softening the sharp acute angle with a soft curve (gray lines in the following illustration) of additional clay before making the mold. As you already have your mold cast, it would involve the careful carving away of plaster on those interior angles to produce soft curves, leaving possibly 1/8" or thereabouts of the acute corner still showing on the design face of the mold. This will give you a good reference then, for carving the added clay away from the area, resulting in the interior acute angles that you desire... once the cross is pulled from the mold... while the clay is still moist and easy to trim.

 

 

And as Chris suggested, 'Your slip make-up, timing in the mold and drying times are crucial...'

My outlined solution, should help eliminate some of the stress on the cross and possibly from you dealing with the problem, though it will involve a little more hands-on labor for each item cast... the trimming away of extra clay! I've seen many commercial molds that have included areas to be trimmed, etc., so it really isn't an alien idea in slip casting to do additional 'finish' work. Good luck!

[tluvs2create]

Wow, wonderful, thanks so much and thanks for all those great illustrations. I will definitely give this a try.

 

BUT, take a look at this photo - any idea why these cracks would form? There were no cracks when I pooped it out of the mold. The cracks only seemed to form after a day or so of drying. Also, seems the cracks are right in the areas where the last of the drizzles of slip were poured. Any thoughts would again be very much appreciated.

 

Thanks so very much!

[AmeriSwede]

My first observation does confirm what you stated... one can see the repetition of concentric cracks that follow exactly the same pattern as left by the slip.

 

Not having personally witnessed anything like this before, I am a bit stymied ... I would be inclined to think that there was certainly some uneven drying taking place along these pouring trails. A couple of things that I would look at in determining the problem...

 

• How does the total thickness of this general area (front to back) compare to the rest of the tile? Thicker, thinner, about the same?

• If this was the first casting, possibly the mold was too dry, which can really suck moisture out too quickly. To alleviate this instance,

it is often beneficial to lightly spray mist the surface prior to pouring. I have read of some that actually recommend a light spray mist

followed by a fine dusting of talc, though I've never actually done that.

• When this last trailing of surface slip was poured, was the slip underneath already drying and set?

• Are the cracks visible on the face side as well? If not better results may be attained with a quicker pouring, since it is a rather thin

piece. A little skill development with that one.

• Are you using commercial slip or your own mix? Is it mixed to a smooth homogenous consistency and does it have just the right

amount of deflocculant? Have you checked the specific gravity?

 

Those of some of the first questions that I would seek answers to in order to correct the problem. Good luck!

[tluvs2create]

Wonderful thoughts and comments. In answer to them;

The tile is fairly even in thickness all over, it just has a slight relief. Yes, this was the first casting in this mold (well first after weeks). The last time I used it, I press molded it. The only reason I tried slip pouring it was to see if it would act in the same way as the cross and the larger tile. Trying to see if it was the slip or the mold most likely causing the problem.

 

I mixed it up real good (shook it and stirred it) and poured it quickly. No, it wasn't dry or even setting up when this last bit of slip was poured - at least as far as I can remember - unless I came back later and saw it needed a little more after some drying had taken place. To tell you the truth, at first thought I would say no - but as I think about it more, I may have added some more later after it was starting to set-up. Honestly, I can't remember.

 

 

I am using a commercial slip from Amaco. I called to ask them about it and they said I was the first to complain of this problem. She mentioned using a release agent on it and was going to have me talk to the tech. guy. I haven't had a chance to talk to him yet. It seems to be very smooth and consistent throughout.

 

I will try pouring another all at once with no additions at a later time - perhaps that is what I did - just can't remember.

 

Thanks so much again, very, very insightful.

[AmeriSwede]

Wonderful thoughts and comments. In answer to them;

The tile is fairly even in thickness all over, it just has a slight relief. Yes, this was the first casting in this mold (well first after weeks). The last time I used it, I press molded it. The only reason I tried slip pouring it was to see if it would act in the same way as the cross and the larger tile. Trying to see if it was the slip or the mold most likely causing the problem.

 

I mixed it up real good (shook it and stirred it) and poured it quickly. No, it wasn't dry or even setting up when this last bit of slip was poured - at least as far as I can remember - unless I came back later and saw it needed a little more after some drying had taken place. To tell you the truth, at first thought I would say no - but as I think about it more, I may have added some more later after it was starting to set-up. Honestly, I can't remember.

 

 

I am using a commercial slip from Amaco. I called to ask them about it and they said I was the first to complain of this problem. She mentioned using a release agent on it and was going to have me talk to the tech. guy. I haven't had a chance to talk to him yet. It seems to be very smooth and consistent throughout.

 

I will try pouring another all at once with no additions at a later time - perhaps that is what I did - just can't remember.

 

Thanks so much again, very, very insightful.

 

 

 

 

Two notes here on this, your last posting....

 

I earlier noticed a few air bubbles visible in your second posted photo.... this is no doubt caused from shaking the slip prior to pouring. The slip should be gently stirred so as not to include air into the mix, for best results. This may take a little more time but the payoff can be more successful pieces.

 

The Amaco rep that you spoke to 'mentioning the use of a release agent...' , this could possibly be the light dusting of talc as I mentioned previously. I would forgo the use of that unless the other fixes warrant additional assistance.

 

Ideally, in making the mold, if I desired a finished thickness of say 1/2" then making the model should include an additional percentage of added height (maybe 1/4"- 3/8"), which would allow me to pour just one time. The shrinkage of the greenware and from the firing would be compensated for by the additional 'reservoir height' of slip.

 

When the cast tile appears firm enough to remove, place a plywood board or piece of sheetrock on top of the tile/mold, flip this over and gently but firmly slam down on the table surface and the tile should pop right out. May take a little practice, but once you see how much force it takes it becomes quite easy and natural....

 

Good luck!

 

 

[Natas Setiabudhi]

A friend suggested I try slip casting with the tile molds that I have made, especially a larger more detailed one. I have to admit, I loved the ease of it. However, I am having problems with cracking. What am I doing wrong? I have gone through a whole gallon of the stuff. My first cast was a larger 8x8 detailed tile (with somewhat high relief in the center). This one cracked a lot in the center. The outside areas of course firmed up quickly far quicker than the center, so I left it the dry for sometime. When I returned, it was dry enough to pop out but had a lot of cracks in the center. So, I tried it again and kept my eye on it - still took some time for all parts of the tile to dry enough to pop out of mold and was developing cracks in some areas while other areas were still very wet. So, figure this was too large and detailed a tile to mold. So, I tried a cross shape that I had made, smaller thinner and simpler. However, this one dried quickly in the center and quickly formed cracks again before the outer portions of the cross could even set up. So, I figured that was just not a good design either for casting. Next I tried one of my fairly simple, 4 x 6 low relief tile. This one seemed to dry nicely - no cracks, popped out of mold and then left to dry on drywall board then sandwiched between drywall board. During this drying process it developed cracks in the center and near the edges. These cracks BTW look like the kind you see if you had pudding in your frig. un-covered for too long - not hairline cracks at the edges or anything.

 

So, what am I doing wrong? Is there anything I can/should be doing to prevent this or should I give up on slip casting all together? Thanks much for any help you can give.

 

 

 

Hello, may be i just sharing of my experiences. Now, I'm doing a lot of slip casting of my works. My opinion anything can be made by molding. In my country, all of recipe of body or glaze must be made by self. So We must think creative to solve that. Recipe for body, especially for casting is the proportion of plastic (clay, china clay), non plastic material (feldspar, quartz, whiting, etc.), deflocculant, and water. May be your recipe of body too much water or deflocculant. If the piece is hard to dry, may be the deflocculant is too less. Also a moment of lifting a piece from the mold is very important. May be you're late lift the piece. Or may be you try solid casting technique.

 

Good luck

[tluvs2create]

Thought things were going well. Popped out of mold with no cracks, day one drying no cracks then the fateful day - day 3: This is what I find:

Seems the cracks follow the flow of the slip as I was pouring. Is it the slip, perhaps? Again this is commercial - Amaco. I have no clue as to how to make my own slip - seems pretty complicated with measuring gravity etc.... Really didn't want to get into all that.

[AmeriSwede]

Sorry to say, tluv, I've never witnessed anything like this before.

 

 

Viewing the photo, I can see that there are the two cracks, the center core and the larger almost completely uniform concentric crack circling the interior, equidistant from the inner crack and the walls of the piece. That implies to me that it is most certainly a problem in how the piece is drying.

 

My reasoning tells me that as the interior portion of the tile is shrinking the exterior portion is binding (probably through too much friction) which may be preventing the entire piece from shrinking uniformly, thus pulling the casting apart (cracking).

 

My first attempt at remedy would be to place a couple sheets of newspaper on top of the piece and then the plywood/sheetrock panel before flipping it over and removing the cast from the plaster mold (onto another panel of plywood/sheetrock). This would allow the piece to have a couple layers of newsprint under it while drying. After the first day, I would then place five or six sheets of newsprint (for padding) on the detailed face side and then a plywood/sheetrock panel on top of that and gently turn the cast over so that it was face-side down.

 

At this point I would transfer it to a wire rack (like that in your oven) by placing the wire rack on top of the casting, sandwiching it between the wire rack (on the back side) and the newsprint layers and plywood/sheetrock (front side) and flipping it over. Then remove the newsprint and plywood/sheetrock and allow it to finish drying. The wire rack, touching the piece only in linear points and allowing air to dry both sides uniformly, should offer less resistance to the forces of shrinkage.

 

Also, at about day one (or day two), once it has set up enough to carve on prior to flipping onto new newsprint, I would take a ceramic loop tool (similar to that pictured below) and gently level the thicker, raised edges to the flat plane of the back, so the whole piece would be more uniform in thickness. That would be my preference.

 

Good luck

[Natas Setiabudhi]

May be I'll give you advice from different angle. You don't have to make your own body, there is a simple way. You just add other raw material to commercial slip for modification. I think your slip is too fluid, therefore add china clay or feldspar, 10 to 30% in dry state to your slip. This mixture needs stirring by mixer machine for homogenous. You can use usual electric drill, but you need special drill for it's head (you can ask to the ceramic supplier). I hope, this post not to make you confuse...

 

Good luck