Reduction firing at 3am.

Here few shots during the reduction firing around 3am. It’s always amazing when you see a kiln during firing, specially at night when it’s dark and when it’s so quite all around: the magic world of clay, the magic world of firing and, of course, the magic atmosphere of the surroundings.

First look while Pietro is removing the bricks door.

After a first look while Pietro was removing the bricks door, we started the “human chain” to unload the kiln and start the discussions about the pieces just fired.

Discussion (b) and works.

LITTLE SELECTION OF SLIPS FOR SALTGLAZE

Any oxide, or mixture of oxides, or any glaze or body stain (or their mixture) may be used in saltglaze slips, with interesting results.

Dry Oranges, Yellows and pinks

Try: Mixtures of various Ball clays and China Clay.

Good results – for example:

AT Ball Clay. 20

China Clay     80

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AK Ball Clay  40

China Clay     60

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SMD  Ball Clay   50

China Clay          50

————————-

Porcelain          75

China Clay        25

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Fireclay         20

China Clay     80 (fierce orange)

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Porcelain Clay 20

China clay 10

Tin oxide 3

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Porcelain Clay 98

Illmenite 2

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Browns:

Shiny Brown: Red Clay  50

BBV Ball Clay 40

Flint 10

Experiment with Red Clay and China Clay mixtures + Iron oxide.

Pure White slip:

Hyplas 71 ball clay 25

China Clay       25

Flint 35

Cornish stone 15

Rich  Glossy or Fluid Oranges

AT Ball Clay 60

Potash Feldspar 20

Whiting 10

Flint 10

Rutile 10

Red Iron Oxide 5

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SMD Ball Clay 75

Grolleg 25

Vanadium Pentoxide 15

Titanium dioxide 10

Tin Oxide 10

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AT Ball Clay 63

Nepheline Syenite 32

Flint 5

Zirconium Silicate 5

Titanium dioxide 10

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Base slips for colorants to be added

China Clay              30

SMD ball clay         40

Flint                         15

Nepheline Syenite         15

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Nepheline syenite 50

Hyplas 71 Ball Clay 50

Colorants:

8% Rutile – pearl – tan

5% Titanium dioxide – pearl

0.5 %-3% Cobalt Carb. – blue

6% Rutile + 3% Cobalt – green

6% Manganese dioxide. – brown

!0% commercial stain can be added to the recipe

To encourage break up in salt glaze as used by Daniel Boyle

Vanadium Base slip

China Clay 55

Potash feldspar 25

Quartz 15

Bentonite 5

Vanadium Pentoxide 10

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Top slips

China Clay 17

Ball Clay 19

Flint 24

Nepheline Syenite 31

Standard Borax frit 9

-

Titanium Dioxide 9 for moonshine white

Or

Cobalt Carbonate 3 for blue

Raw Lining Glazes

Nepheline syentie 1

Soda Feldspar 1

AT Ball clay 1

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Nepheline syenite 33

AT Ball Clay 33

China Clay 33

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Potash feldspar 1

Cornish Stone 1

Hyplas 71 Ball clay 1

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I hope that this is clearer

very interesting recipes I will try some of them out when I do build the new salt-kiln. I might even try soda in the new kiln or I will wait for next opportunity.

I am adding here some of mine recipes which I have worked with in salt. I am also adding my method of salt firing and I hope that it is of an interest. I have written this information for John Mathieson’s book called ‘Techniques Using Slips’ which has just been published. So I thought it might be interesting for La Meridiana participants. Some of the recipes are same as the ones already published on your blog the firing  method is also quite similar, but do take a look.

Hope that this is of some use.

LITTLE SELECTION OF SLIPS FOR SALTGLAZE

Any oxide, or mixture of oxides, or any glaze or body stain (or their mixture) may be used in saltglaze slips, with interesting results.

Dry Oranges, Yellows and pinks

Try: Mixtures of various Ball clays and China Clay.

Good results – for example:

AT Ball Clay. 20

China Clay     80

AK Ball Clay  40

China Clay     60

SMD  Ball Clay   50

China Clay          50

Porcelain          75

China Clay        25

Fireclay         20

China Clay     80 (fierce orange)

Porcelain Clay 20

China clay 10

Tin oxide 3

Porcelain Clay 98

Illmenite 2

Browns:

Shiny Brown: Red Clay  50

BBV Ball Clay 40

Flint 10

Experiment with Red Clay and China Clay mixtures + Iron oxide.

Pure White slip:

Hyplas 71 ball clay 25

China Clay       25

Flint 35

Cornish stone 15

Rich  Glossy or Fluid Oranges

AT Ball Clay 60

Potash Feldspar 20

Whiting 10

Flint 10

Rutile 10

Red Iron Oxide 5

SMD Ball Clay 75

Grolleg 25

Vanadium Pentoxide 15

Titanium dioxide 10

Tin Oxide 10

AT Ball Clay 63

Nepheline Syenite 32

Flint 5

Zirconium Silicate 5

Titanium dioxide 10

Base slips for colorants to be added

China Clay              30

SMD ball clay         40

Flint                         15

Nepth. Syenite         15

Nepheline syenite 50

Hyplas 71 Ball Clay 50

Colorants:

8% Rutile – pearl – tan

5% Titanium dioxide – pearl

0.5 %-3% Cobalt Carb. – blue

6% Rutile + 3% Cobalt – green

6% Manganese dioxide. – brown

!0% commercial stain can be added to the recipe

To encourage break up in salt glaze as used by Daniel Boyle

Vanadium Base slip

China Clay 55

Potash feldspar 25

Quartz 15

Bentonite 5

Vanadium Pentoxide 10

Top slips

China Clay 17

Ball Clay 19

Flint 24

Nepheline Syenite 31

Standard Borax frit 9

Titanium Dioxide 9 for moonshine white

Or

Cobalt Carbonate 3 for blue

Raw Lining Glazes

Nepheline syentie 1

Soda Feldspar 1

AT Ball clay 1

Nepheline syenite 33

AT Ball Clay 33

China Clay 33

Potash feldspar 1

Cornish Stone 1

Hyplas 71 Ball clay 1

The Use of Slips in salt-glaze.

Salt Glaze is formed when ordinary salt or soda is introduced into a kiln designed for this purpose. It is usually a flame fired kiln. Salt or soda is usually put into the kiln after 1240 C, when it immediately vaporise.  The sodium from the salt or soda combines with the silica present in the clay’s surface and creates sodium silicate which is the glass on the surface typified by it’s orange peel appearance. The glaze on the shard will form in salt firing regardless of whether or not there is a slip or glaze applied. However, salt glaze potters use slips to gain colour, but also to alter and manage the surface of the wares. Slips rich in silica will attract more sodium and will promote richer and glossier surface whereas slips higher in alumina will resist the attack of the salt and will create a surface which is matter and dryer. The application of the slips to wares intended to be fired in a salt kiln is much the same as in any other firing, slip can be poured on , brushed on, sponged, sprayed, slip trailed, splashed.  What is different is the fact that the slip decorated pot in salt glaze needs only be once fired. For glazing the internal parts shino glazes or other raw glazes are usually used.

Packing and Firing

Salt vapour is present everywhere within the kiln. Therefore the pots are usually placed on salt resisting wads, which prevent them from adhering to the kiln shelves as everything within the kiln gets a layer of glaze. Like with all firings it is advisable to place tall pots at the bottom and shorter pots at the top of the kiln to promote better flame flow.  Distance between pots is also important for the surface forming. The salting in a loose pack will be more even, on the other hand close pack will promote flushing and variation of salt build-up on individual pots.

When raw firing caution must be taken in the early stages of the firing that pots do not explode. To begin with the firing will be just the same as any biscuit firing in a gas kiln. Starting to reduce quite heavily after 1000 C for 20 min then keep a moderate reduction until 1150C when the atmosphere in the kiln is cleared and oxidised for about 30 min. The temperature will rise and at approx 1200 C reduce the kiln again and maintain medium reduction till 1240 C when the atmosphere in the kiln is cleared and oxidized again ready for  starting to salt. The amount of salt will vary in accordance with the age of the kiln. New kiln will need 1kg for 1sq foot or more. Salt is often introduced in paper wraps shoved into the flame port, whereas soda is introduced in a form of water solution sprayed into the kiln through various spy holes.  Salting process will naturally reduce the kiln so when salting is finished the atmosphere needs to be cleared again. In my firings I prefer to fire to cone 9 and have cone 10 still standing when I finish the firing. In the end all ports are opened for about 20 minutes to half an hour, this process is called crash cooling. Crash cooling not only prevents the crystobolite build up and dunting associated with this, but also has a positive effect on the colour development.  This is my firing cycle however others might vary.

Colour in salt-glaze slips.

Traditional oranges and browns are achieved by using various combinations of clays and it is the ratio of alumina, iron and silica that create these colours.

For colours from oxides any oxide can be mixed in a white base slip to get a result. Most common is of course cobalt which produces blue. This is often too harsh a blue so some iron oxide can beaded to calm it down, If used in combination with iron baring ball clay turquoise is produced. Cobalt in combination with rutile will produce iridescent green, it will also produce green in combination with Chrome, with titanium will produce pearl blue. Combination of Cobalt, manganese dioxide and iron oxide will produce slate colours. Titanium in a base white slip will produce silver pearl effect, while Rutile in the same will be more towards gold. All colouring agents will work one way or another in the salt kiln and their combinations are endless.

With the ceramics technology advancing there are also currently available on the market ceramics stains which will not burn out in the salt firing providing that the kiln is fired no higher than 1280 C. So most untraditional colours to salt glaze such as pastel reds and yellows are achievable from some commercial stains now.

Layering two slips of different vitrifying temperatures as is practiced by Daniel Boyle will encourage the top slip to break up and reveal super exaggerated salt-glaze effect.

The result of salt glaze slips however is almost never guaranteed to be exactly the same as this depends on many circumstances such as type of the base clay, application, the kiln stack, age of the kiln, amount of salt attracted etc.

Clay Bodies

Any stoneware clay body can be fired in salt kiln, however clays rich in Iron will fire very dark and are not therefore good as a background for coloured slips or glazes. Also clays high in alumina will not produce a good result. There are clays on the market specially formulated for use in salt.

But here is couple of recipes for old salt favourites.

Famous Doulton salting body

BBV ball clay 62.5

China Clay 18.75

Potash Feldspar 18.75

Harry Davis Salting body

Hyplas 71    74

China Clay   24

Cornish Stone  1

More vitreous version

Hyplas 71  75

China Clay  12.5

Nepheline Syenite 12.5

These recipes are without filler – grog, molochite or sand can be added to them

Pietro sent me the file with all the recipes you guys used for the soda firing workshop. You can download it and add more recipes, if you want, and see that everything is written correctly. And, please, post this again if you change something! Thanks

RECIPES SODA WORKSHOP 2010

In my case the experiment will start immediately after building new wood kiln with the chamber for soda firing as the one I have right now is not suitable for soda firing (it is soft brick and need to last another year for our woodfiring).

For now I can share the results from the firing in La Meridiana:

All pots fired in gas kiln, reduction, approximatly to Seger cone 8-10 (1250 – 1300°C), soda introduced after cone 8 partly down.

TEST 1

Clay tested: Polar VV – from  H+K Lubná (http://www.kerlubna.cz/) – first picture show the clay not fired and second show this clay fired in woodkiln to 1250°C

Slip tested:

Results:

TEST 2

Clay tested: cca 1/2 M11 salt-Wittgert / 1/2 Stoneware-St.Amand

Slip tested:

Gail Nichols Rutile

Caolin                  80

Nepheline Syenite         10

Quarz                               10

Bentonite                          3

Rutile                                 3

Borax                                 3

Note: Slip applied in different thicknes

Results:

TEST 3

Clay tested: Polar VV – from  H+K Lubná (http://www.kerlubna.cz/)

Slip tested:

Note: Slip applied quit thick

Results:

TEST 3

Clay tested: 1/2 shamot clay for shiedel chimney (the inside tube)  / 1/2 Polar VV – from  H+K Lubná (http://www.kerlubna.cz/)

Slip tested:

Results: Slip applied quit thick

unloading the kiln.

Here all together ready to unload the kiln… the human chain!

Discussion.

And, of course, we started a new discussion about the results obtained in this firing. For sure the temperature inside was right as you can see here:

Cone.

And after all the considerations we prepared the new works for firing and then the kiln was loaded again.

Loaded

Opening the big kiln for the first fire.

Here for the first time we fired the big kiln and there we were looking at the all the pieces!