Jump to content

Recommended Posts

Hi,

   We have been doing the jigger for the first time and the pieces whatever we are making have a cracks in a same pattern in a same location, and we are using Terracotta clay to make the pieces and here I m attaching the cross sectional pieces of the molds., and in the molds I have few doubts and  I would like to get clarified so that I can reduce the cracking doors one by one.,

 Does the thick and thin portions of the molds absorbs water differently so that the pieces are cracking ?

As you can see the picture of the pieces the cracks are happening in the place where the side wall is connecting with the curved edges, and the cracks are happening only with our terracotta clay and we have tried quite a lot of receipes to avoid cracks but nothing is helping out and the cracks are happening in the inside of the pieces and the outside wall is just fine. Also we have tried jiggering few pieces with our stoneware clay and we havent have any cracks in the stoneware.

Does the uneven wall thickness is the reason to get the cracks ?

Wi only the Terracotta is cracking and not the stoneware ?

Wi the inner wall is cracking and it didnt extend to the outer wall ?

Does it anything to do with the plasticity and the composition of the clay?

As far as the design is concerned, we make sure theres no undercuts in the design and even the wall is not striaght and it has an angle of 10% for easy releasing., we used to have few issues with our clay body which  I was working with Tom (Glazenerd) to solve the issue, but apart from that I would like to know what are the other possible reasons for the cracking. 

 

WhatsApp Image 2018-06-08 at 12.30.08 PM.jpeg

WhatsApp Image 2018-06-08 at 12.30.12 PM.jpeg

WhatsApp Image 2018-06-08 at 12.30.14 PM.jpeg

WhatsApp Image 2018-06-08 at 12.30.18 PM.jpeg

WhatsApp Image 2018-06-08 at 12.30.11 PM.jpeg

Share this post


Link to post
Share on other sites

I think it may be the clay body you are using more than the wall thickness or a combo as the wall at that point in mold is thinner the body stays wetter there. How about with bone dry molds does this happen then???

Share this post


Link to post
Share on other sites

If it's not happening with stoneware, I'll suggest that the absence of grog in the earthenware makes a difference in the evenness of drying. Your form is jiggered in a way that creates thick walls next to thinner walls and the stresses are showing where it cracks, with the more dense earthenware being less forgiving. 

Share this post


Link to post
Share on other sites
14 hours ago, Mark C. said:

I think it may be the clay body you are using more than the wall thickness or a combo as the wall at that point in mold is thinner the body stays wetter there. How about with bone dry molds does this happen then???

Yes its happening with the bone dry mold as well :-(

Share this post


Link to post
Share on other sites
11 hours ago, Rae Reich said:

If it's not happening with stoneware, I'll suggest that the absence of grog in the earthenware makes a difference in the evenness of drying. Your form is jiggered in a way that creates thick walls next to thinner walls and the stresses are showing where it cracks, with the more dense earthenware being less forgiving. 

Rae, yes we have try adding 5-20% grog to the body but still the cracks are happening in the same area. :-(

Share this post


Link to post
Share on other sites
1 hour ago, Babs said:

After removal from mold how are you trying your pots?

Babs,

  The crack is happening in the mold itself but it not a end to end crack and its a surface cracks but bit deeper than the usual surface cracks.

Share this post


Link to post
Share on other sites

As Mark C says, it looks like the clay in the thickest part of the mold, at the transition from bottom to side, is drying most slowly. If you don't want to shave the mold, maybe extra compression with a rib in the problem area before the clay dries enough to pull away from the mold? Is the earthenware taking a lot longer to dry than the stoneware? Perhaps it needs to be in a wetter state than stoneware to jigger well??

Share this post


Link to post
Share on other sites

Clay used for jiggering is typically softer than clay used for throwing. Jiggering exerts shear stress; which I think is the case shown in your sample pics. Uneven wall thickness also indicates (to me) that the jiggering arm  is struggling  to push the clay up  the side walls. 

a. Increase the WOPL of the clay body to 26-28% to soften it.

b. Use the recipe I emailed you.

laterite: the Frankenstein  of pottery clay!!

T

Share this post


Link to post
Share on other sites

Well maybe a bit less jiggering and a bit more jollying:-//

When working my C6 clay I can get away with being a bit less subtle when rolling and pressing.

My terra cotta needs handling with care aka shortcrust pastry as opposed to pizza dough.

So rolling and pressing into forms requires more gentle rolling  and easing into shape more carefully or the clay WILL crack .

Longer all over drying too.

Share this post


Link to post
Share on other sites

Its your clay body more than the mold. Try some additives to clay body.

I just fired some friends jiggered plates and jiggering requires all things to be right-the clay bode the mold and the feet placement.

In my friends case the feet where not in right spot (out far enough for support) and the cone 10 porcelain warped.He was used to eathenwares which warp far less at cone 06.

Edited by Mark C.

Share this post


Link to post
Share on other sites

Reading hamer and hamer last night, reclaiming clay here..bentonite a lubricant for sure and increases plasticity But not workability! Well I was reading this as some of my reclaimed clay lacked strength, my description, ok for thick walled thrown pots but didn't hold up to my usual thickness of thrown pots. 

What would a better plasticiser be? And would this poster's problem be similar?

 

Edited by Babs
Clarity

Share this post


Link to post
Share on other sites

Well go for it glazenerd, I can skim read pretty well :-))))) but interested in the logic.

I once had a batch of clay which totally dried out and on smashing it to reconnstitue I found strands of stuff a but like dried latex.

Share this post


Link to post
Share on other sites

Babs:

sodium bentonite absorbs up to 15 times it's weight in water. Ball clays absorb water as well, but that is expressed as WOPL. (Water of plasticity). Example: 100 grams of OM4 ball clay requires 35 grams of water to produce a pliable ball. When sodium bentonite ( not calcium bentonite) is used as a plasticizer in clay: it only absorbs 1/3 to 1/2 of the water it can actually hold. Ball clays do the same, but not on the scale of bentonite. Then when you throw, both continue to absorb water: which in turn weakens the mechanical properties of the body.

point 2: potters tend to keep adding ball clay or bentonite to their clay recipes so that it comes out of the pugger all creamy, dreamy, and plastic. The problem with that is 1: the higher the % of bentonite/ ball clay used to produce that result: the moe water it will absorb when you throw it: resulting in mechanical failure- slumping walls. CEC ( cation exchange) is the chemical reaction that results in plasticity. CEC has a rate law: so much exchange per 24 hour period until maximum exchange takes place. Usually 3-5 days. I use a ball clay with a CEC of 11.0 at 8% of recipe: which produces more plasticity than 15% of OM4 which has a CEC of just over 7. However, it is short when it comes out of the pugger, but five days later it is plastic. Not FAT, but plastic. 

A simple way to judge excess  plastizer is the amount of cream on your hands when you throw. If it creams and oozes through your fingers, way too much plastic materials. Which also means it will get weaker the more you wet it and handle it on the wheel.

point 2. The finer the particle, the higher the green strength. Ball clays with particle sizes under 0.50 microns, have MORvratings around 700. MOR= modulus of rupture: a testing method used to determine green strength. As particle sizes increase, MOR values decrease. Porcelain is known for its fragile green ware state: but kaolin runs 1.4 to 2.5 microns typically. ( there are blends lower and blends higher in PSD.

lastly, if you are finding white fibrous patches or granular areas in your dried reclaim: I would put Nep Sy as the flux used in the body. Nep Sy has 14-20% soluble salts that do migrate as the clays dry. Ten plus years ago when clay used Minspar, G-200, or other potash as flux; pinhole issues where fewer and farther between. Now that Nep Sy is the body flux of choice; pinhole issues are common.

t

Share this post


Link to post
Share on other sites

Thanks glaze Nerd, certainly is logical. 

The stuff I found in the dried out clay was stretchy and 'rubbery strands.

What are the plasticisers used for grout etc by builders.

My current terracotta clay co as with a central core running through the purged clay of a darker colour than the outer clay.     Bentonite??? Rich clay?

Thanks for keeping it succinc!

Share this post


Link to post
Share on other sites

Babs:

i would have to look through the mine data from Aussie land to narrow down specific clay varieties. However, from dealing with Gokul on the laterite from India: red clay species contain hematite ( iron). From the world mine maps, many locations in Aussie also have hematite. Because hematite is FeO3 and not FeS which is normally found in the States: it has a direct reaction to oxidation. (Exposure to air) (IF) your red clay has hematite, then the exterior of a pug  will be a different color than the interior. Hematite is also known to streak black to dark grey: and typically fires a deeper terra cotta color than in the States.

as far as the fibrous/ gummy strands: very possible  your bentonite is not dispersing.

http://shop.walkerceramics.com.au/BA455.1/Hectalite-_dash_-White-Bentonite/pd.php

this product sold down under is mislabeled: it is sodium hectorite, not bentonite. Sodium hectorite is much more plastic than bentonite, and is a much finer particle size. It does gel to some degree, but not nearly as bad as bentonite. Would also make a better glaze suspender than bentonite.

one simple trick about dry mixing in a pugger: I dry blend the recipe first, without  the plasticizers. The  week or day before mixing; I mix the plastizer with a few pounds of the dry clay and dry blend. Then I add the water and let it sit at least a day before I mix it. The plasticizers then absorb all the water they are going to before mixing. 

T

Share this post


Link to post
Share on other sites

Thanks. The purged clay dark cores I am referring to is a commercial clay. And one batch of this when dry had the rubbery strands which made me think a product had been added which was not bentonite or ball clay...

I am aware of the light Bentonite, curt filled me in in that.

 

Share this post


Link to post
Share on other sites
On 6/17/2018 at 4:23 AM, Babs said:

Reading hamer and hamer last night, reclaiming clay here..bentonite a lubricant for sure and increases plasticity But not workability! Well I was reading this as some of my reclaimed clay lacked strength, my description, ok for thick walled thrown pots but didn't hold up to my usual thickness of thrown pots. 

What would a better plasticiser be? And would this poster's problem be similar?

 

Babs, I have been try using the synthetic plasticiser to the clay and can share you the results once the clay is ready.

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

×

Important Information

By using this site, you agree to our Terms of Use.