Questions about plasticity and natural clay

[roderick]

Hi all, I'm based up in Finland and we have a lot of very clean local clay deposits. It's grey and fires to a dark red brown at the top end of it's range which I would guess is around 1100C looking at the glow. (the test we did was in a self built wood downdraft kiln without any cones or thermocouple. It started to boil and melt) There was a brick works on the site where I got the clay up until the '50's, so I know the clay is usable in some form or another. There is a LOT of clay around here, I guess as a result of glaciation?

I dug a heap of clay about 15 years ago and it's been wrapped up outside getting deep frozen every year (-40C) I dried and washed a bucketful a few years ago which has been stored indoors since then.  After hand wedging the clay it has a good smooth consistency but it feels quite strange. It almost like a gel??  It cracks and breaks as soon as you try to stretch it, but when you make a large ball of it, it starts to sag. If you manipulate it very slowly, say by squeezing, it deforms readily without cracking? It is not sticky at all, although in the bucket it looks like it should be as there is a fair bit of surface water on the clay.

I don't have much experience with raw clay but I guess this needs an addition of a plastifier like Bentonite or Ball Clay or something?

So a couple of questions... 

Should the clay be dried and powdered first? Or can I just work out the water content and wedge the appropriate amount of plastifier into it?

....and what percentage of plastifier would be a good place to start?

Thanks in advance

Rod

 

[Babs]

Sounds like it is very thixotropic...loosens up consederably when worked.

Sounds like may be low firing if used for bricks.

There are quite a few posts in these forums on plasticity and properties of clay .

Do a search.

Hope Glaze Nerd is around!

Personal message this guy

Glazenerd.

[glazenerd]

16 hours ago, roderick said:

It's grey and fires to a dark red brown grey color usually indicates magnetite iron source.around 1100C It started to boil and melt) earthenware clay 13-15% alumina, with 7-8% iron, low silica  45-50%?  Melting and boiling at 1100C /. 2012F?  Firing wood means carbon, which does not play well if high sulfide content in the clay. The iron/sulfide combination in a reduction atmosphere would lead to early glass formation at 1750-1800F ( 1000C) 

however, I have other concerns about this early melt. I checked Finlands' mineralogical survey; lead is commonly found. Lead melts early and is fluid. Second concern: cryolite is also found. Google it. Melt at low temperatures and also creates fluidity and will boil at higher temps. Sodium and potassium melt at 2012 & 2044F respectively (1100C) range. So you are melted and boiling at 1100C, which means potassium and sodium are not responsible, nor is calcium. These are concerns, but most likely early glass formation from the iron.

 It is not sticky at all, although in the bucket it looks like it should be as there is a fair bit of surface water on the clay. sesquioxides like to hold water on the clay surface- one way to tell if your clay has them. Cracking is another way.

I don't have much experience with raw clay but I guess this needs an addition of a plastifier like Bentonite absolutely not or Ball Clay or something? you need a ball clay with 27-29% alumina content. A CEC ranging between 7-8, or a WOPL of 33-34 at roughly 20% addition. In street terms, a medium plasticity ball clay: 80% raw clay, 20% ball clay. 

So a couple of questions... 

Should the clay be dried and powdered first? yes.Or can I just work out the water content and wedge the appropriate amount of plastifier into it? no

 

Thanks in advance

Rod

 

Tom

[Mark C.]

Most local clays are low fire-I just fired some river clay for a friend-it was a good teracota color at cone 08 and melted and bubbles into a lava mess at cone 10.

This is the usual story. You will need to add refractory materials  to raise the temps. Most local clays I have seen are pretty plastic already as the partical size is uniform as well.

Thes clays require much work(additives) to make into workable bodies for potters . 

I did this in collage as a project to make a local clay work at cone 10..

my 2 cents

[Magnolia Mud Research]

45 minutes ago, Mark C. said:

Most local clays are low fire-I just fired some river clay for a friend-it was a good teracota color at cone 08 and melted and bubbles into a lava mess at cone 10.

On the other hand, the natural red-orange clay from my ponds, when fired to cone 10-11 in reduction, is about as "mature" as the commercial soldate 60 high fire clay body is at the  cone 08-06  bisque stage.   When ground fine, and soaked a few hours, the clay makes a smooth slip and an ok  clay body for handbuilding; have never used it for throwing so have no data.   

I use my pond clay  for surface decorative purposes at cone 10  and as an source for color  in high fire porcelains.   

LT

 

[roderick]

Thank you everyone for your replies, especially Tom.
So I have a bucket of dried clay that I found in the shed. It has been frozen and thawed and frozen again for a number of years and is now in a dry and flaky state. 

I have some Hyplas 71 Ball Clay but as I am more one for street terms I don’t really understand the chemistry of it all :o) 
I guess this will be OK to use...

From Digital Fire
Devon medium strength low iron ball clay
Oxide   Analysis   Formula
CaO      0.10%   0.010
MgO      0.30%   0.040
K2O       1.80%   0.104
Na2O      0.30%   0.026
TiO2       1.60%   0.109
Al2O3   18.75%   1.000
SiO2       70.50%   6.381
Fe2O3       0.80%   0.027
C   0.20
LOI   5.30
Oxide Weight   512.18
Formula Weight   541.99

How important do you think it is to sieve the dry clay once I have crushed it a bit? It was a pretty clean slip until it got frozen and left.

Tom, this you may find interesting. Goes a bit over my head...
http://www.airihortling.fi/The%20color%20changes%20with%20earthenware.pdf
...and an online version without the photos?
http://www.uiah.fi/kll/hortling.html
...and about lead in Finnish clay
http://www.uiah.fi/kll/research/lead.html

Thanks again for all your advice. Will have a go at this next week sometime.
Rod
 

 

[roderick]

.....another link for Tom

http://www.airihortling.fi/Lead release in glazes.pdf

[glazenerd]

Rod:

from table 1 in your link: Finnish clay is 50% silica, 18% alumina, 9% iron.  In addition, 9% LOI and 3% carbon (discussed later.

i saw nothing about lead or cryolite content, so that answers those concerns.  Those addressed, your molten pool would be caused by low alumina, further compounded by the early reduction of iron (1750F/ 1000C) in a wood firing atmosphere. The 9% LOI would indicate sulfide/ sulfate content, coupled with 3% carbon. Carbon would be a direct indication of lignite coal particles. Sulfur/sulfide/sulfate content which is high in Finnish clay is directly involved in reducing the iron (9%), which is a powerful flux in reduction.  Firing in a reduction atmosphere (wood) further compounds the early reduction of iron.

you need to slow down the firing cycle to 60C an hour from 600 to 1000C. I have not done a wood fire, so one of our forum members who have that expertise will have to chime in on this issue.

The second issue is the low alumina content. 18%. Mark C recommended  the addition of refractory materials which usually consists of 10-20% kaolin; which has 37% alumina content typically. His recommendation is correct, however I recommended  20% ball clay with 27-28% alumina content because you have a plasticity issue. I was addressing the refractory and plasticity issue with one fix. The ball clay you linked will not work, because it is likewise has low alumina. However, you could blend a combination of 70% Finnish clay, 20% of the ball clay shown in your link, and 10% kaolin.  If you have a glaze calculator; use the analysis for Finnish clay shown in table 1 of your link as the data in the calculator. Try various blends of Finnish clay, ball clay, and kaolin until you achieve 21-22% alumina content by weight or a SiAL ratio of 5:1. Remember you need the ball clay for plasticity, but also the alumina in kaolin to raise the PCE of the clay.

Finnish clay has 9% iron, so diluting it with ball clay and kaolin will still give you a reasonably high iron content. One of our forum members who we nick named Proud Mary started a thread in Clay & Glaze Chemistry called " Hudson River Clay". She chronicles her journey of working with a local clay with an iron content of 8.4%. You will find many similarities to your journey.

Tom

[glazenerd]