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Making Glaze From An Assortment Of "free" Raw Materials


Chilly

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As some of you may recall, I inherited several buckets of "raw materials for stoneware glaze" from an old, retiring potter.  The advice received on this forum, when I asked "what can I do with them?" was to give them back or get rid of them asap.

 

The raw materials that I owned were: Bentonite, Nepheline Syenite, Mag, Alumina, Black Ball Clay, Feldspar Potash, Talc, Quartz, Dolomite, CC China Clay, and “No label†which on testing, turned into a blue-green glaze at ^8.  I also acquired Red Iron Oxide, Zircon Oxide, Iron Ox – Yellow Ochre and Copper Oxide.

 

First off, I found a recipe for batt wash:  50/50 alumina and china clay.  Tested it out and worked OK.  Nice, as I was running out.

 

Then I used some of the Bentonite in a commercial glaze.  To stop it settling out, if I recall.

 

As for the rest of the buckets, well, I'm not known for taking advice if I don't like it.  So I read everything I could find about making your own glazes.  Problem I found with all the books was they seemed to think I already knew more than I did, and they didn't explain things from the bottom up.  I began to see the light after watching the John Britt video where he describes the three main things needed for a glaze and likened them to driving a car. 

 

When I got to the point of knowing I needed a glass former (silica) to make the glassy surface, a flux to make the silica melt at a lower temperature than it wanted to melt at, and a refractory to stop the glaze from sliding off the pot I felt I was getting somewhere.

 

But, according to the books lots of raw materials contain more than just silica or just a flux, they contain bits of everything.  If I could just go to a shop and buy “silicaâ€, “flux†and “refractory†life would be easier.

 

Looking at my materials, I realised I already owned a silica - quartz, but none of my recipes used it, they needed flint.    I owned two fluxes - neph sy and potash feldspar, one of which was in my recipes.  Finally I needed some refractory - whiting was used in most of the recipes.  Plus I needed china clay which I had. 

 

The other materials would have to wait.  The potter I acquired them from was into matt ^10 glazes, which explains the dolomite.  No idea yet what I will use the black ball clay and talc for.  Nor what kind of mag I have, or what to do with it.  As I also got some kiln shelves, lots of props, a bucket of terracotta clay and a half-gallon of commercial clear ^06 glaze I was happy happy.  Plus lots of wooden bats for the wheel I've since declined.

 

I finally settled on one recipe to test and tweak.  I particularly wanted to find a nice white glossy glaze to use on the inside of mugs, and wasn't happy with the commercial variety from a local supplier.  

 

All the (UK) recipes I found that used some of my materials also required Whiting and Flint, so bought a kilo of each of those.

 

Step 1

Find out if what it said on the bucket was the same as in the bucket.  I made a test tile with dimples (film canister sized) and put a blob of each dry material into a dimple. Marked the tile and wrote down what was in which dimple.  Fired to ^6, but kiln was over-firing, so more like ^8. Re-read every book, and it seemed that each bucket was indeed correctly labelled.  Also by then I’d been collecting wood ash, so tested that too.  (The writing on the test tile is in pencil, added after firing, the numbers were scribed at leather-hard.)

 

Raw%20Materials%20Test%20Tile%20a.jpg

 

Step 2

The first recipe I chose to test/tweak is a clear glaze maturing at 1250°C (from the book “Creative Pottery†by Peter Cosentino):

 

Feldspar Potash     40

Flint                      30

Whiting                  20

China Clay            10

 

Zirconium Oxide    12

 

The tweaks I've made are:

Swap Neph Sy for Feldspar Potash and call that “Aâ€. 

Swap Quartz for Flint and call that “Bâ€

Swapped both and called that “Câ€.

 

I then found another, similar, clear recipe in “Teach Yourself Pottery†by John Gale:

 

Feldspar Potash     48

Flint                      10

Whiting                  20

China Clay            22

 

Zirconium Oxide    12

And I repeated the swapping as above.

 

Finally I found an Ash Glaze in Cosentino:

 

Feldspar Potash     40

Ash                         40

China Clay              20

And repeated the swapping as above, both with and without the 12% Zirconium Oxide.  (I didn't mean to try this with the ZO, but forgot and put it in, so had to mix another batch without it.)

 

 

The problems I've had with the commercial white glazes is cost, pin-holes and crawling?  see photo below:

Jo%20Mug%20Crawled.jpg

 

I made a few flat test tiles, but realised to find out exactly how my new glazes would react on the inside of mugs.... I'd have to make test mugs.  14 small slip cast mugs later (I used the same clay that I use for throwing (not very well) and hand-building, turned it into slip with the addition of water and sodium dispex.

 

I also made some new test tiles, based on one of John Brit’s designs, but made from rolled clay, incised with a pattern and then left to dry over a rolling pin so they are kind-of “S†shaped.  The will be fired over round kiln props so they don’t sag.

 

If the mugs come out OK I'll re-fire them with an outside glaze and use them.  If not OK, hey-ho.

 

Step 3

The mugs and samples were fired in the centre’s kiln and emptied on xmas eve.  Cones 6 and 7 are down and 8 is almost too, so the kiln is still over-firing.

 

Three of the mugs are cracked (dunting?), the rest are not.

 

There is still some crawling, but not as much as the commercial glazes, but the glaze is not white, nor are any of them particularly glossy.

 

Sample%20Mugs.jpg

The two mugs on the left are new commercial glazes, first time tested.

 

Taking each original recipe and it’s changes, I've not really come to any conclusion, except that maybe some were applied too thin and not evenly enough.  The two ash glazes (with ZO) are not suitable for inside mugs, they look like they need a higher firing.

 

I think the two that are the most suitable are the recipes with Neph Sy instead of Feldspar Potash, but they're still not what I'm looking for. 

 

So, how do I improve on them?   I'm not sure what to try next.  I guess Tin Oxide instead of ZO should make the glaze more white than cream.  But with the raw materials I have, and the fact that no-one else at the centre is firing ^6 it will be a while before I have another kiln load to test.  

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Lots to take in here, but:

Flint is silica.

Whiting is calcium carbonate, a flux.

The refractory is typically alumina, which can be sourced from many different glaze materials. China clay, or koalin, is the most common source, although is it also present in feldspars.

 

One thing that will help you is to read about the raw materials and what they provide to the glaze. Many glaze materials provide several important ingredients, not just one, and that's where it can get confusing sometimes.

 

A couple of good exercises to do are line blends and triaxial blends with fluxes. I have my students do blends with koalin and flint added, so they get some useable glazes. For instance, for a line blend they would blend:

 

A:

Flux A 75%

EPK    15%

Flint    10%

 

B:

Flux B  75%

EPK     15%

Flint     10%

 

Investing in a glaze calculation app will also help to make things more understandable.

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I'm impressed that you've got this far Ann. I also acquired a collection of raw materials from another potter, but haven't got round to mixing and testing my own glazes yet. I also looked into the basic glaze formulation info, which helped my understanding when I found it! I tabulated all the ingredients I'd found, so that I had a reference for which of the constituent parts they contributed to the glaze, and then added notes about them that I came across. The Digitalfire website is really good for the chemical information on everything - I have a shortcut on my iPad!

I almost feel as if I must stop making anything for a period of time if I'm going to get my head round mixing my own glazes, it seems to take so much time, especially when every aspect requires the assimilation of new knowledge.

I found an excellent glossy white glaze, but probably not high firing enough for you - Potclays 2248 Glossy White Zirconium whch fires up to 1120 oC according to the spec, but may fire higher. They may do something similar for stoneware.

Hope you find what you're looking for! Celia

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In your melt tests the quartz looks too melted to be quartz, when I fire quarts to cone 8/9 it is this very dry crusty layer that has not melted at all. More like the flint underneath it. As Neil said, flint, quartz and silica are very interchangeable terms in ceramics. I think real flint comes with some impurities along with the silica.

 

I knew I had this picture somewhere. This is my melt test with soda feldspar, potash feldspar and cornish stone along the top. Quartz then china clay on the left. The other 6 are 50/50 mix of each feldspars with quarts and each feldspars with china clay. Look how dry the quartz is.

post-23281-0-25282000-1420585241_thumb.jpg

 

I am also a bit suspicious of the mag which I assume you mean magnesium oxide. Never had any myself but digital fire says ' Magnesium oxide is very refractory, and is used in the manufacture of bricks and crucibles for the metal processing industries, thermocouple insulation, etc.' I wouldn't have expected it to melt but I could be wrong. EDIT ;- just reading my own document and it does say 'at higher temperatures, it is an active alkaline flux' so it could be MgO and I just don't know what I am looking for.

 

Talc is also a source of magnesium with silica Mg3Si4O6 Dolomite also has magnesium with a bit of calcium CaCO3.MgCO3 so I am still not sure what the Mag is.

 

It is also interesting to note the flashing that Sodium can bring in the neph and wood ash. I really like that slight orange burnt look around the edge of my glazes. A soda feldspar would look similar.

 

One thing I find good to do is although the chemistry will not be exact, enter your recipe into http://glazecalculator.com/ and have a look at these limit formulas to see how your values stack up. It will only be an estimate but then you can edit values and see what changes they make in the glaze chemistry. Add in recipes from books and see what values they get.

 

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I love digital fire. It is a good place to start looking into rocks and their chemical makeup.

 

Those glazes look like a good start anyway. Keep it up :D

 

Here is a doc I have on major oxides and what they do https://docs.google.com/document/d/1uNNIyESjMwv4tUn50A6lhbRquP3cGWqR9hlYBZMWjAM/edit?usp=sharing

post-23281-0-39881000-1420576643_thumb.png

post-23281-0-25282000-1420585241_thumb.jpg

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Hey, I know that glaze!

It's not glossy, because it's underfired. I use it A LOT in cone 10 reduction. The bowl in the foreground of my avatar is your feldspar recipe with epk substituted for the China clay, and small additions of tin and ochre for colour. I'm actually in the middle of reformulating it because it crazes.

 

You can download a free trial version (good for 2 months) of Insight from the Digitalfire website that will help you enormously to compare what different substitutions of materials will do to a given glaze. They also have video tutorials that you can watch, and they'll walk you through how to use the software to fix common glaze problems. This is how I spent my Sunday afternoon. It was actually very helpful. I am a nerd, and I need a life.

 

Part of what you will find confusing is the fact that a couple of your ingredients contribute more than one important component to your glaze. Feldspar for instance, contains both flux and refractory alumina in significant amounts. This is why straight subatitutions of one ingredient for another can get complicated, and having a computer do the math for you to see how it works in theory is so very nice. It gives you a much better place to start testing.

Some notes on your feldspar glaze, to save you a bit of time:

 

As I mentioned, it crazes. Feldspars have a high COE and neph sye is worse. Subbing the neph sye in will create crazing headaches. I have five dollars that say your cracked mugs are your neph sye blend.

 

I have found that you need to add very little opacifier to this glaze to get a white surface. 12% is certainly too much, and can contribute to a rough underfired surface. It will also pick up any hint of iron in your clay body, so if you aren't using porcelain or something closely related to it, it won't be a true white.

 

Application is critical. It shows all the drips.

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