Archive for the ‘VAPOUR GLAZING’ Category

17 PMpWed, 28 Apr 2010 23:31:59 +000031Wednesday 2010

SORRY – I have just seen my last entry with the recipes all oddly spaced. Not very helpful, so I am trying again just the recipes this time.

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

Nepheline 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

————————-

I hope that this is clearer

17 PMpWed, 28 Apr 2010 23:00:58 +000000Wednesday 2010

Hello from Mirka in UK,

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

RECIPES.

17 PMpWed, 28 Apr 2010 19:30:50 +000030Wednesday 2010

Hello to everyone!

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

Tests, which call for further explorations and experiments

17 PMpWed, 28 Apr 2010 12:38:38 +000038Wednesday 2010

As we have tested many local materials (clays from all over Europe, brought by the participants) we have learned quit a lot about their look in soda firing. We have also seen the wide possibilities, which surely will lead to more experiments.

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:

Nepheline Syenite 33
Kaolin Sedlec Ia 33
Silica 33
Fe2O3 1,5
TiO2 13,5

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:

Kaolin 40
Witgert 40
Borax 5
Ultrox 10

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:

Kaolin 40
Witgert 40
Borax 5
Ultrox 10

Results: Slip applied quit thick

New tests!

17 AMpFri, 23 Apr 2010 08:54:54 +000054Friday 2010

Here Pietro taking care of the firing.

Pietro and the Soda.

17 PMpThu, 22 Apr 2010 23:36:22 +000036Thursday 2010

Hello All,

very interesting pictures and fantastic work.

Please could your publish in the blog the dimensions of your soda kiln and the size of the kilnshelves?

Thanks

Mirka

Kiln wash

17 PMpWed, 21 Apr 2010 12:45:36 +000045Wednesday 2010

Hi Mirka,

just contact Peter Meanley, he wrote an article for Ceramics Technical some years ago about his tests with kiln wash and bricks. I ordered his preferred material (Pyruma 1a, made in Leeds by Everbuild Building Products LTD., Site 41 Knowsthorpe Way Cross Green Industrial Estate, LS9 OSW Leeds) some days ago and will test it in the next firing with the new kiln.

Peter Meanley
6 Downshire Road
BT 20 3 TW Bangor County-Down
Nothern Ireland

0044-2891466831

All the best for the new kiln!

Markus

Hello form UK (By Mirka GoldenHann)

17 AMpWed, 21 Apr 2010 10:22:36 +000022Wednesday 2010

Hello!

I have lived in the UK for 17 years and have never felt like I was on an island untill now.

Anyway – the photos on the blog look great and I am sorry I am not there. I have a few questions though. Your kiln – what is the internal lining? What are the bricks, are they coated with a wash? How does it deal with the corosion? It looks from the photos like an altered gas-kiln? Where is the soda entered into the kiln (in how many places)?

My existing slat kiln is 1.5 m3 stacking space – so it is a quite big kiln. I had it for 10 years and it is on it’s too eroded to carry on using. I will be building a new kiln in the summer and I am considering a much smaller design. I want to know what are you experiences from La Meridiana using smaller kiln in terms of hot and cold spots?

Also the kiln shelves – they look like silicon carbide shelves? I have no experience working with those and I have heard and read various reports, but I would like to know, if your kiln shelves are silicon carbide, your opinions and experience with them. Are they worth the extra expense?

Also I would like to ask – could you publish a photo of this kiln when the soda is introduced. I am very curious to see if there are any visible vapors like it is when salting.

Oh, all these questions.

Here is me with my salt kiln:

Here is me with my salt kiln – making fog for Dorset.

And some work…

All of the below was fired in this kiln. I am mad about colour in salt-glaze.  Some of these glazes are raw and some applied to biscuit.

The plan was to try some of these glazes  in soda. Well – next time.

VAPOUR GLAZING notes by Pietro Elia Maddalena

17 PMpSat, 17 Apr 2010 21:35:24 +000035Saturday 2010

Introduction

The choice of a particular way of firing can influence dramatically the expressive options of the ceramic artist. In vapour glazing the essential transformation brought about by heat and vapours is seen as part of the creative process. This chemically energetic activity can produce varied and exceptional graduations of colour, texture, pattering and flashing in the clays, slips and glazes. It has been the appreciation of such effects that has inspired renewed interest in this firing methods.

Gay Smith

Gay Nichols

Historical notes

Vapour glazed stoneware, generally once-fired, develops glazes and colours by introducing common salt or soda carbonates in the kiln at a temperature when the clay is maturing. Both salt and soda carbonates at those temperatures will decompose and the sodium, as a vapour on it’s way to the chimney, will react with the silica and alumina of the clay to form a thin, slightly orange-peel textured glaze which is very hard and resistant to corrosion.

Salt glazing developed in Germany and it went out of favour for tableware in the second part of the 18th century, its use confined to cheap jars, bottles, and drainage materials. Several craftsmen potters since the 1960’s have revived this technique, which produces a very integrated and interesting variable finish.

Vapour glazing using sodium carbonates is a relatively new approach to vapour glazing, having first emerged among studio potters in the 1970s . It has since proven to have far more potential than the reproduction of “salt glaze imitation ”, offering new direction for ceramic work.

CLAY notes by P.E. Maddalena

17 PMpSat, 17 Apr 2010 21:34:17 +000034Saturday 2010

Most clays may be vapour glazed with a various degree of success. As a basic principle the more silica in the clay the smoother the glaze will be, but with less silica and more alumina, the more distinctive “orange peel” texture will result.

The colour of the clay after firing is governed by the nature of the firing and by the iron content. From pale gold (1%) to pale tan (1.5%) to medium brown (1.8 -2 %). More than 3% of iron will cause the glaze to be progressively duller.

Not all clays are well fluxed by the soda because many contain insufficient free silica, e.g. china clays, ball clays and stoneware clays with alumina/silica molecular ratios between 1 to 2 and 1 to 4. The best clays for vapour glazing are those fireclays which are high in silica. These can have alumina/silica ratio of 1 to 5 or higher.

Extra silica is sometimes added to clay as quartz sand but silica in isolation does not make a glaze, some alumina is necessary. Small amounts of calcia and magnesia help to stabilise the glaze.

When the kiln is soaking at high temperatures, various forms of quartz develop in the body of the pots: some of this quartz is very unstable, the most unstable form being cristobalite.
As already mentioned, the more silica in the body, the better it will salt. However, the more silica in the body the higher the danger of cristobalite to develop in quantity and pots to shatter unless due care is taken.

The most stable of quartz forms is mullite which forms if the kiln is cooled very rapidly. Therefore fast cooling down to around 1000 °C has developed thus stabilising the pernicious effects of excessive silica.