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cf66

Calculating Different Shrinkage Rates

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Hi all,

 

I'm new in this forum and would appreciate some wisdom on figuring out shrinkage rates.

I'm working with Laguna B-mix Cone 5, which as per Laguna has:

 

Avg. Shrinkage 2±%: 12%

Avg. Water Absorption 1± %: 2.3%

COE x 10-6: 5.74

 

(note: no idea what COE is. Do you?)

 

Now, what exactly is this 12% shrinkage? Is it the shrinking after both bisque and glaze firings? (I bisque at cone 06, glaze at cone 5). What is the shrinkage from wet to bone dry then?

Does this mean, for example, that if I want the drain hole in the sink I'm trying to build to be a finished size, that is, after glaze firing, to be 3.175 cm (which is 1 1/4", I'll use cm since it's simpler for calculations, being decimal and all...), at WET (plastic) stage the hole will need to measure 3.60 cm, where 3.175 cm is 88% (100% - 12% shrinkage rate) of 3.6 cm? Am I figuring this correctly?

 

Where does the 2.3% water absorption rate come into play? Is that the shrinkage that happens from plastic/wet stage and bone dry?

 

If the posted avg shrinkage rate is 12%, how much of that shrinkage happens after the bisque firing and how much after the glaze firing? What happens if, for example, I need to do a second glaze firing, will it shrink even more? And what happens if that second glaze firing is not at the same temperature as the first glaze firing, let's say it's for decals at cone 012?

 

I need to be really accurate for the pipes and hardware to fit correctly on this sink.

 

I'm confused. I'll thank you in advance for any light you can shed on this for me...

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Thank you so much, Norm!

That's great information and not so great news to me, who hates getting into the whole chemistry/engineering aspect of it all, but alas, seems unavoidable!

Now I have A LOT more to think about and learn...

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Water absorption is the amount of water the fired clay will absorb or how vitrified the clay body is after firing to its maturity.  Porcelains and some other stonewares can have an absorption rate of less than 1% at maturity; other clay bodies, higher than that, with earthenware generally being the most porous or absorbent with rates 10 to 14%.  It is an indicator of whether your fired piece -- wheel thrown or hand-built -- will absorb water. 

 

The absorption rate you cite is pretty good for functional ware -- wheel or handbuilt -- and likely good for a sink that will have standing water in it as part of its use.  For functional, I try to avoid clay bodies with absorption rates of more than 2 1/2% percent . . . I do a lot of vases and water holding vessels and the lower the absorption, the more I am comfortable telling customers whether or not the vase will seep or leave damp marks on wood table tops. 

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Shrinkage rate is rated as the total from wet to maturity in the correct cone firing. So, yes you need to add 12% to your desired drain hole dimension? It is always a good Idea to do your own shrinkage test by measuring a tile with 10 cm, fire in bisque and measure and fire again to maturity and measure.

Absorption tests are done after the maturity firing.

 

Marcia

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cf66

I have made many a sink-I suggest 1st you find the drain hardware (brand  you like) and have that 1st so you can measure all the flanges and pipe diameters

2nd you throw a few small bowls and cut the holes to what you think will work taking in all the measurements for shrinkage. Also think about glaze taking up space as well as making the drain recessed so no standing water is around flange.

Mark down these tests on paper so you can recreate them

Fire these tests a try your drain to see how it fits.

After all this then you can make the full sized sink and know what will work.

There has been a thread or two on this so can also search them up on this forum.

Mark

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So, yes you need to add 12% to your desired drain hole dimension?

 

 

 

c'mon people! Don't add 12%. You will get a wrong result.

 

100 + 12% of 100 = 112

112 - 12% of 112 = 98.56

 

cf66, your calculation with % is correct.

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No worries Marcia :)

With small objects and large tolerance it's probably not important.

But why add another "maybe" to the relatively unpredictable clay... drilling a hole or grinding the edges to fix it later is too much risky work.

 

1-0,12=0,88 and then 3,175/0,88=~3,61 is just so much easier.

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I guess it really depends on what the clay really does. Do the manufacturers figure 12% shrinkage one way or the other? I really don' t

Know. I have been doing shrinkage tests for almost 50 years and it has worked for me. That is all I can say about it. I understand the math comes out differently when you subtract the shrinkage from the original size. For very specific almost machinist accuracy, you can go to kinko's and reduce the measurement on their copy machines. No math required.

 

marcia

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I was thinking of contacting John Pancini at Laguna to ask him, but The two you mention will do.

The frustration people get when the same clay, lid and pot shrink differently really happens. Stress? Water .

Clay has properties that are much different from metal or wood. You just have to keep working with it.

Marcia

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Not sure whats up with all this talk-just forget inches switch to metric-its So much easier-I did this so many years ago it may have been the 70;s glaze/body calc class makes %s very easy

Cut a pice from a pug about 3/8" thick 1 1/2" wide and about 6 inches long from whatever clay bodies you want to learn about

Mark 10 MM with needle tool-thats two lines at 10 mm apart-keep this piece flat and bique fire it-measure agian and now its shruk whatever % from the 10MM write that on the bar in iron or colbalt then galze fire to whatever your are firing to and recocd this-write this on the bar along with the clay body type-Hopefully you put a hole it for hanging-now you have all the info needed to make another lide to bisque or glaze state.-Marcia spelled it out but I want it to be really claer and easy-metric turns it intoo a easy %.

I use all measurement with metric rulers in clay its so much easier.

When I make a lid i just use my wet finger to write underneath the lid the measurement in even metric #s so when that lady a few weeks ago broke her lid at an AZ show I asked her for the numder underneath she said 14 which means 14 mm wet-I threw anouther mailed it off after glaze fire fits perfect every time sight unseen.

You need to know your bodys specs if you plan on working well with them.

I'll get my formula off the wall in am and shot a few bar photos so you can see these simple things- I leaned all this from some fres Alfred grads fom the laste 60's they learned t from guys like Randal and Rhodes-some may have even heard of these guys but you will not find them on UTube.Better to look in the Library for thier books.

This is all very basic and must be done early if you want to amke covered forms or whatever fit well.

Mark

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Guest JBaymore

Norm hit that +/- 2% figure before I had a chance to.  Plus or minus two parts out of twelve is is a HUGE margin of error.  This reflects the impacts of both the consistency of the amount of water added to the various production batches as well as the impacts of differenig forming methods.

 

For precision ceramic work, industry uses "clay" bodies.... that have no actual clay in them.  Makes them much more predictable.

 

best,

 

.............................john

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Also, how wet is the original piece you measure?  Is it very wet or just slightly wet.  Every bag of the same clay is supposed to be the same weight, but it might have dried a bit in the despatch/delivery/hanging around in the studio stage.

 

Is this why pottery is an art not a science?

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Guest JBaymore

Norm,

 

Wow... your studio situation sounds like a NIGHTMARE!

 

best,

 

....................john

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Here's some shrinkage bars of various porcelains Sasuga and MC porcelain which at one time I had made for me in a slip business.

I wrote on the bisque #s with colbalt and clear glazed over it -these are easy to make and you learn right away about your clay body.

These are cone 10 bars

I have written on the wall for my Daves Porcelain 12% shrinkage

Finish size divided by .88= wet size

 10 inch  plate is 11.3 wet

 

These numbers are eay to figure once you know your rates and use a metric ruler.

Mark

post-8914-0-58160000-1388447415_thumb.jpg

post-8914-0-58160000-1388447415_thumb.jpg

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I get my clay form Witgert. They give you shrinkage for few temperature ranges per clay type. Really helpful stuff when precision is important.

We make moulds and build prototypes for designers who like to use ceramics (clay and/or porcelain) Loads of fun, if you ask me.

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Hi all,

 

OK, so just when it seemed all clear and resolved, turns out my test tiles of the same clay body at bone-dry stage have shrunk at different rates (yes, same clay body)!!

I have made 2 to 3 tiles of for each of the 4 clay bodies I wanted to test, each is a slightly different thickness and size.

For example, my three Laguna BMix cone 5 tiles have shrunk 6.25%, 6.8% and 6.7% respectively.

My two Laguna Steve's White cone 04 tiles have shrunk 5.4% and 4.83%!!

 

They're now being bisque-fired, we'll see how it all ends...

Any thoughts, anyone?

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Did you stretch or roll the differently-was the clay from several pugs are relaimed or all the same pug?Mosture will matter at the unfired state they are in for your measurement-this dry state (bone dry) will vary depending on shop moisture conditions.

I only record bisque fire and glaze fired measurements-more stable.

Are you measuring between the 10cm fine line marks?

 

One last thought-ceramics is just when you think its all worked out it throws you a curve ball-the vary nature of this clay deal.

M

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I used a rolling pin to make the slabs (turning the slab 90 degrees each time, so they got rolled in all 4 directions). Each set of tiles came from one single piece of clay cut from one single bag for each clay body, on the same day (within less than an hour for all 4 different bodies and pieces thereof). I even compressed each tile with a rib after rolling them (now not 100% sure I compressed ALL of them but for sure some). And yes, I am measuring between the fine lines, some I even made 20 cm long in an attempt for better shrinkage representation/accuracy.

And agreed on the curve-ball aspect of ceramics!!  :wacko:

 

God help me at the time of making my actual product. Given this, I now basically can't understand how any mass production of identical items is possible, even though it clearly IS possible as proven by the billions of identical mass-produced ceramic items that exist in the world!! :P

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