Engobe And Glaze

[Babs]

On 3/14/2019 at 9:06 AM, glazenerd said:

BUT, unlike cars; when your clay breaks down there is no shop to take it to.

Yeh then I go to the rabbithole dwelling glazenerd or back to reading the bbooks etc (After I take out Murphy rainwater and the Babs factor.)

Last one usually is the problem..not acting with that commonsense. :-))

[njabeid]

On 3/16/2019 at 7:06 PM, njabeid said:

I've got the clay blend, with 58% sandy clay instead of 60. I have started throwing with it, and it holds up really nicely but I find it a bit too sandy and definitely short, kind of woolly or fuzzy when trimming. I had previously tried a blend of 50% river clay, 40 % white and only 10% sandy, and it is much smoother. More work ahead, firing them and trying them with the suggested engobe.

I'm not really happy with this blend - pity because I mixed a fairly large batch. It exudes clouds of fine slip, too, and slumps a bit. First time a mug handle cracks right through. :-/ 

 I'm going to tweak this batch to about 40 percent white and 30% each of the sandy and the river clays, see what it's like. I didn't add bentonite because everything was already wet and mixed, and I wanted to try it without any, then add some to compare.

Question: having read the info about COE and shrinkage etc., can one assume there is a correlation between them, so if I want to fit an engobe or a glaze to a clay, is it OK to run the latter through a calculator (Insight) and compare the COE with the shrinkage test in %?  Slow processing all these cations and stuff.

[Min]

COE figures from glaze calc software, like Insight, don't work for some glazes, all slips and all claybodies. COE figures for glazes are useful when all materials in the glaze have melted into a glassy matrix. Crystalline glazes, including microcrystalline, have some materials that have precipitated out of the glassy matrix therefore are not part of the complete melt resulting in COE figures that won't be accurate. Slips and claybodies are on the extreme end of this principle. Shrinkage is just that, how much a clay shrinks during drying and firing, COE is the distance any material expands per unit of length upon heating one degree of temperature. COE is reversible as on cooling there will be contraction.

[liambesaw]

Calculators are ballpark figures for coe, I use them like this: glaze 1 crazes on my stoneware, and has a calculated coe of 7.5, so I know I should adjust the recipe to lower the coe.  If the craze patterns are tight, try to adjust it down to 6.5, if it's barely crazed, maybe drop it to 7.0

I don't know the coe of my clay because it's a commercial clay, so I'm kind of stuck doing back and forth estimates like this.  

[njabeid]

Min, of course. Hope blinded me, misled also by Glazenerd's mention in the Understanding COE thread : "A clay body with a COE of 6.00 will shrink 6.00% from peak temperature to room temperature. A glaze with a COE of 9.00 will shrink 9.00% from peak to room temperature". In my mind "contraction = shrinkage", but I forgot about the /° part of it. It's so odd that last year's engobe suddenly started blowing up on any clay (starting last year by the way). It was all going well and now I'm a bit lost. A lot of wasted (except for the pedagogic value) time and effort. Thanks everyone for the help.

[njabeid]

14 minutes ago, liambesaw said:

Calculators are ballpark figures for coe, I use them like this: glaze 1 crazes on my stoneware, and has a calculated coe of 7.5, so I know I should adjust the recipe to lower the coe.  If the craze patterns are tight, try to adjust it down to 6.5, if it's barely crazed, maybe drop it to 7.0

I don't know the coe of my clay because it's a commercial clay, so I'm kind of stuck doing back and forth estimates like this.  

I have learned to do that, particularly as my clays aren't even commercial, so completely unknown, and I fire to 1,120°C for which there are practically no glaze recipes out there. That works with glazes, but this engobe has flummoxed me. Now one of my trusty glazes has crawled all over a small piece with no engobe, particularly where thin (??) and I can't figure it out. :-(

[Min]

The clay COE figures that some companies put out are likely in the ballpark but the validity of the figure is dependent of firing conditions. If you have a base glaze recipe that you can change the proportions of, without adding another flux, to get a few versions of it to go from low to high expansion then try them out on the clay with no engobe. One variable at a time. Glaze COE figures get you in the ballpark but if you use dissimilar recipes, ie different fluxes, both having the same COE it doesn't mean they both will fit. Some fluxes give more of a "stretch" to glazes than others, others don't work in a linear fashion, like boron for example. The clays I use don't come with COE figures either.

[njabeid]

My glazes are fine.

I tried the same engobe one more time with a different clay, and it was OK after bisque, although the engobe's bond was irregular and messed up the stencil - some bits came unstuck .

And then this, after 1,120°C :

I'm going to try now with Glazenerd's engobe recipe and see whether it's any better.

[glazenerd]

Nancy:

shrinkage is used by potters primarily to predict how they need to form custom pieces to match once shrinkage occurs. From the clay side: shrinkage from wet to bone dry will give some insight of clay composition: finer particles absorb more, so they shrink more, etc. fired shrinkage also gives insight: carbon content, molecular moisture, flux levels- all play into fired shrinkage. 

I use Glazemaster instead of Insight; it allows me more flexibility. Glazemaster also has a clay formulation module, which I find useful- most will not. It also allows me to write my own formula limits: which I have done for porcelain- will finish stoneware this year. I also have the benefit of imported recipes, with dilameter tested COE figures.  

I realized your desert clay was sandy, apparently it is real sandy. It would take an extra step, but the courser  particles could be removed.

Tom 

[Magnolia Mud Research]

Nancy,

is the sand you’re using a “sharp “ sand or a smooth sand?  Smooth  sand particles are more likely to slump in green ware than sharp sand for the same size particles .  May also effect movement in final firing.  (I leave Tom to explain why, I as must leave for an appointment).

LT

[njabeid]

35 minutes ago, Magnolia Mud Research said:

is the sand you’re using a “sharp “ sand or a smooth sand

Same as for concrete. I really don't know, I haven't looked and the grains are really small. I'll try to see in daylight.

[njabeid]

1 hour ago, glazenerd said:

I realized your desert clay was sandy, apparently it is real sandy

It really is, coming from way out in the Sahara, but the river clay is sandy too. Not worth trying to get the sand out, it's quite hard enough to screen it for rocks! The white clay contains tiny sharp stones, maybe flint. The river clay might make good manure! Thanks for all the input.

 

1 hour ago, glazenerd said:

shrinkage from wet to bone dry will give some insight of clay composition

Interesting :

Shrinkage test tiles 100 mm

White clay: after bisque @ 1000°C 95 mm, after firing to 1,120°C 90 mm, i.e. 5 & 10%

River clay : 90 and 87 mm, i.e. 10 & 13%

Desert clay : 97.5 and 95 mm, I.e. 2.5 & 5%

Your suggested blend : 94 and 91.5 mm; I.e. 6 & 8.5%

My blend : 92 and 87, i.e. 8 & 13%

Yours shrinks less, but mine is nicer to make. More testing ahead.

[glazenerd]

2 hours ago, njabeid said:

It really is, coming from way out in the Sahara, but the river clay is sandy too. Not worth trying to get the sand out, it's quite hard enough to screen it for rocks! The white clay contains tiny sharp stones, maybe flint. The river clay might make good manure! Thanks for all the input.

 

Interesting :

Shrinkage test tiles 100 mm

White clay: after bisque @ 1000°C 95 mm, after firing to 1,120°C 90 mm, i.e. 5 & 10% - these rates would be in line with fine particle earthenware: with lower levels of natural fluxes. Higher flux levels = more fired shrinkage. It also says molecular water is minimal, as well as organics. Organics burn off= higher LOI, = higher fired shrinkage. 

River clay : 90 and 87 mm, i.e. 10 & 13%  10% shrink from wet to bone dry= higher percentage of ball clay! bentonite! or hectorites. The more plastic a material is= the more water it will absorb= more dried shrinkage. Remember the river clay was- heavy. Only 3% fired shrinkage concerns me. Low fired shrinkage = higher % alumina! iron! and the absence of natural flux. River clay might need to go bye bye.

Desert clay : 97.5 and 95 mm, I.e. 2.5 & 5%  2.5% dried shrinkage= nearly zero fine particle- nearly zero ball clay or other plastic material.  2.5% fired shrink = high alumina, high iron! large particle.

Your suggested blend : 94 and 91.5 mm; I.e. 6 & 8.5% acceptable, but it gives me concern about iron/alumina levels.

My blend : 92 and 87, i.e. 8 & 13% more acceptable: indicates higher level of plastic materials.

Yours shrinks less, but mine is nicer to make. More testing ahead. what shrinkage means to potters, and what it means - is not the same. 

Sharp = more surface area. 

Being an engineer- here is COE in your terms. Steel expands and contracts at one rate, concrete expands and contracts at another: structures incorporating both for support, weight bearing, span, etc.- require attachments that allow for that movement. All COE is doing is bringing differing expansion/ contraction rates closer together,  

now factor in solids: seismic waves transfer differently through solid ( concrete) and soft surfaces ( wood).  At certain temperatures: the solids (glaze and clay) go through molecular earthquakes as they heat or cool.: and how those molecular movements transfer through clay and glaze is different. I believe the engineer term is elasticity- modulus of rupture also measures elasticity.

tom

[njabeid]

Not a mechanical engineer, but understand. Thanks again for all the time and knowledge. Light bulbs are going on!

There certainly is a lot of iron in all those clays. They all come out the same colour, except for my previous white clay that is dead white when green, ivory bisqued and buff glaze-fired and vitrified. And all are variable anyway.

 

[glazenerd]

3 hours ago, njabeid said:

 

There certainly is a lot of iron in all those clays.....and vitrified. therein lies my concerns. At your peak temperature, the clay should be in an expanded state; hence the assessed higher COE calculation. If the iron is being reduced, and that is being interpreted as vitrified: the COE is much lower. However, the expansion pattern in prior samples also suggests higher COE clay body. We are about to find out...

 

 

 

[Magnolia Mud Research]

3 hours ago, njabeid said:

Same as for concrete. I really don't know, I haven't looked and the grains are really small. I'll try to see in daylight.

Concrete uses only sharp sand. 

LT

[Min]

If Tom's engobe recipe isn't a fit then another one that would be worth trying is one I got from George Vardy, a tech for Mason Colour Works (Mason stains company). I'm pasting an email he sent me a few years back. If you can't get VeeGum T then I would sub with macaloid (bentone ma) if you can get that or if not then bentonite. In place of the stain use zircopax. Doubt you would need the 40 parts, I would mix up the base, with the bentonite, then try 10 parts zircopax, brush / spray it on a test piece then add another 10 parts zircopax and repeat process up to the 40 parts. 

"One of the best engobe formulas is a very simple one, developed years ago by Ron Mason who then owned Mason Color Works, Inc. Carol Mason, his daughter, now operates MCW.

EPK Kaolin 10 parts

Feldspar.     25 parts

Flint.            25 parts

Stain.           40 parts

Mix well with water, add 1 part VeeGum T that has been FULLY broken down in water, screen thro 100 or finer screen, bring to your painting consistency by addition of more water if needed.

When using very strong stains such as cobalt blues or chrome greens you may need to reduce the amount of stain if the fired color is too strong for your requirements. 

This can be used to decorate on clay or bisque. Be sure to dry fully before glazing."

[njabeid]

8 hours ago, Magnolia Mud Research said:

Concrete uses only sharp sand. 

 

I know about concrete. It's an interesting point in clay. It's much too small for me to see - would need a microscope. It feels kind of non-smooth between the fingers, so maybe It's semi-sharp?

[njabeid]

18 hours ago, glazenerd said:

There certainly is a lot of iron in all those clays.....and vitrified. therein lies my concerns

Why do you object so strongly to iron in clay? It is a flux, right? Aren't all earthenware clays brown-red because of iron? You say if it is reduced it could be interpreted as vitrified. Is that a problem? This is an electric kiln, so theoretically no reduction. The vitrified white clay I had previously broke with a smooth glassy surface, had nearly zero absorption. The batch I bought today has more dark patches, so will have more iron than the 2% that one had, and I'm not expecting any vitrification with all the blends.

[glazenerd]

I object to iron once it crosses the 4.00 molar %.  At which point it crosses into the brittle arena, which in turn destabilizes  the body.

iron can be reduced in oxidation pending sulfur/ sulfide content; also dependent on firing speed.

on page 2 of this link, about half way down is a lengthy discussion on iron/ reduction.

 

[njabeid]

Aha. So much to learn...!

But I fire to 1,120°C.

[njabeid]

On 3/18/2019 at 2:38 PM, njabeid said:

I'm going to tweak this batch to about 40 percent white and 30% each of the sandy and the river clays,

The blend is ready, with a bit less than 30% of river clay. It is gorgeous to throw, let's see how it fires. It seems not to slump, which is my pet peeve with the white clay. 

The white clay is interesting: it's very much like stones, even when put in water overnight, but the sedentary layers delaminate (spontaneously like a book if it is very clean, less so if it's browner), aided by breaking the lumps down by hand. Then  I whizz it all with a mega blender, and it gradually  develops into something like gray satin-smooth whipped cream.

[Babs]

So your Saharan clay could be likened to the Loess clay of Asia and parts of Europe.

[njabeid]

11 hours ago, Babs said:

So your Saharan clay could be likened to the Loess clay of Asia and parts of Europe.

Interesting point. I don't know the Loess clay directly, although I know what it is. I rather thought Loess was windblown, whereas this stuff is dug out of the ground. The Sahara used to be covered in water and vegetation and giraffes, then it dried out, so maybe this clay is water-based or windblown long ago. The town of Chinguetti is built of stone with this golden clay used as mortar and plaster. Beyond the town there are sort of wells going down to the clay layer, and a maze of tunnels a thousand years in the making and said to be kilometres long. Recent research has uncovered a gigantic river that flowed  through the north of the country (where the gold mine is now), and I'm trying to get some of the clay there.

[Babs]

Misunderstanding..though should have guessed, I thought your clay was blown in from the Sahara:-)))) Aeolian clay.