Bonsai Ceramics

Michael requested a lotus pot approximately 300mm x 300 x 12 then showed me an image of a stunning pot made by internationally famous potter, which was his inspiration. “And can I have it glazed in copper red?”

Gulp. ” I will try but make you no promises.”

I threw multiple pots and altered them to create the lotus curves, but was not satisfied with any of them. So I sculptured a lotus pot in solid clay and then made a mould in plaster. It was big in size and took weeks to dry and then I could not lift it.

Months have now passed and the mould is now ready to slip cast the pot. It took another few weeks to dry before it was ready to bisque fire. Finally I have reached step one with no cracks.

Glazing was another challenge as the pot was too big to fit in a clay bucket and an awkward size to pour a glaze over evenly.

I placed the lotus pot in the best spot in the kiln for heavy reduction firing and was anxious an attentive through the 10 hours of the gas firing, starting reduction at 900 degrees C. I then fired through to cone 10, approximately 1300 degrees C.

Another two days for the kiln to cool before I could look and see. Would the kiln gods be kind or would a have a splotchy red pot or a pot with no red at all?

Thanks Michael for the challenge!

The Illusive Grey Glaze

I had a commission to make a  soft grey pot with a “scalloped” rim. I was initially a little daunted by the intricate rim but after drawing multiple templates for the rim I felt I was on top of the task. How difficult can a grey glaze be?

I made two pots. For pot number one, I bought some black stain and mixed it with a white glaze and for pot number two, I mixed back stain in a white underglaze  and covered it with a clear glaze. I was, I now realise, foolishly confident, that I would get one if not two nice grey pots.

Ceramic stains are a mixture of oxides that are melted in a kiln and then ground to a fine particle size. Thus I thought they were reasonably stable. The black ceramic stain  is usually a  saturated loading of one or more  oxides, in this case there was, among other things, some copper oxide and some red iron oxide. So when I diluted it to a  soft grey looking glaze and underglaze my pots  showed  copper oxide traits following a reduction firing.  Yes definitely not grey. Pot one is showing a copper red and pot two a nice little red copper blush on a soft green.

So after a rethink I decided to try- for pot three – a similar process to pot one, with an oxidation firing cycle ie no reduction and with pot four I bought a commercial grey glaze. Still feeling confident I would succeed. Hah!

There is a saying in ceramics ….”test, test, test!” So I now made new glazes and started testing on small test tiles. I found a lovely soft bluey grey and on testing it seemed perfect. I made pot number five, alas the base cracked and as you can see pot number six is a beauty….just not grey!!!

June 2021

Is the primary function of the feet on bonsai pots, aesthetics or  for horticultural function?

Certainly feet are critical for the flow of water and oxygen. Feet add a  little height so wires under the pot are hidden. However most feet can achieve these functions.

The feet on a bonsai pot can be strong and textured, subtle and plain, decorative, or delicate.

So what makes aesthetically pleasing feet on a bonsai pot?

• The feet must flow with the line of the pot
• The feet need to fit in with the design so if the pot is elegant and refined, the feet need to be elegant and refined
• The feet may mimic the rim of the pot or the texture of the pot
• The feet must look like they belong to the pot, not a last minute addition
• The height, the width and the angle of the feet can take a pot from ordinary to beautiful

Feet can be hand made, moulded, turned, carved or thrown.

This dragon foot was hand made and will be used to make a plaster mould and then 3 or 4 feet can be made in the mould  then  added to the pot, either facing up or facing down and curved to fit the pot. One such foot took me about 30 minutes to make so making 4 for a pot would be very time consuming.

These  hand sculpted handmade feet could also be moulded.

These feet were made by throwing a separate ring of clay which is then cut up shaped and added to the pot base.

These feet were turned. After a pot is thrown and dried to the point where it can hold it own weight it is placed upside down on the wheel and the base is shaped to provide feet.

The feet on this pot were handbuilt and carved to create the stepped effect.

These feet were handmade.

So next time you look at a pot take note of the feet and ponder the time and skill needed to add a little flourish.

Denise Allen

A cone is a small pyramid of ceramic material designed to melt when a specific ratio of temperature and time is reached during a kiln firing. This displays the ‘heat work’ on the clay, so you know when the clay is vitrified and the glaze has matured. Cones come in a range of sensitivity from 022 up to 14.

In a glaze firing I want to reach cone 10, which on my pyrometer may read at a temperature of 1260 to 1300 C. The temperature reading is a guide, but the cones are a more accurate measure for what is happening to the clay in the kiln.

I put 3 cones in a glaze firing (cone 8, 9 and 10) and I peep through the hole in the door of the kiln, after I remove the bung, to see if the cones are beginning to melt. Potters use quaint terms like “tipping” and “touching its toes” to describe how much a cone has melted.

‘Heat work’ is dependent on the rate of temperature rise and the length of time the kiln has been firing and strangely enough is also affected by the humidity and barometric pressure. It is a bit like slow cooking meat, the meat will cook at a lower temperature if the cooking time is increased.

Close up of cones after firing

Cones in kiln pre firing

A New Year Challenge.

Starting Feb 1st I aim to post a new bonsai pot everyday for 100 days.

Follow me on Instagram    @  100pots100days

Denise Allen

Type of Clay for Ceramics

Clay is similar to bonsai potting mix in that there is so much variety in the ingredients and their proportions depending on the result you want, and your personal preference.

Most clays contain several different types of clay minerals with different amounts of metal oxides and organic matter.

Clay differs from the inelastic soil and fine sand because of its ability, when wet with the proper amount of water, to form a cohesive mass and to retain a shape. This quality is known as clay’s plasticity. When heated to high temperatures, clay also partially melts, resulting in the tight, hard rock-like substance known as pottery or ceramics. (See the Ping Test post.)

There are different types of clay used in ceramics, and they can be characterised by the  temperature the clay must be fired to in order for it to become mature, or reach its optimum hardness and durability.

The four  most commonly used clay types  are earthenware clay bodies ( fired at 950 to 1100 C), mid-fire stoneware clay bodies( fired at 1160 C to 1225 C), high-fire stoneware clay bodies( fired at 1200 C to 1300 C), and porcelain clays which fire to maturity at about 1800 C.

Various additives can be used to change the  properties of clay, for example additions of ball clay will make the clay more plastic, additions of iron impurities will make the clay speckled and addition of grog ( fired clay that is then ball milled into fine pieces) will make the clay textured and more resilient and some can add crushed volcanic rock such as trachyte which tends to melt and add strong specks and texture to a finished pot.

The type of clay body makes a big difference to the expression of a glaze. A white clay with few impurities will give a stronger, cleaner colour than the same glaze on a darker clay body containing iron and grog where the glaze expression will be more earthy and variegated.

Below is the same glaze -on a white clay and an iron rich clay

If a bonsai pot is made of stoneware clay,  high fired (Cone 10 being an average temp) and free from cracks, it should give a nice clear ‘ping’ when you flick it with your finger. Not as beautiful and clear as crystal but ringing in a similar fashion. To make the pot ‘ping’ you need to hold it in such a way that it can vibrate. Using a light touch, for example if it has two holes then holding the pot with a finger in one hole and a thumb in the other, underneath the pot. Then a flick the pot with a finger on the other hand.

OLYMPUS DIGITAL CAMERA

OLYMPUS DIGITAL CAMERA

OLYMPUS DIGITAL CAMERA

Ceramics pots ‘ping’ as a result of vitrification of the clay in the firing process.  Vitrification is from the Latin word vitreum, meaning glass. As the temperature of the clay rises in the kiln the clay hardens and the spaces between the refractory particles (those particles not affected by the kiln heat), are completely filled up with glass, fusing the particles together and making the clay body impervious to water. This also leads to shrinkage of the clay, usually about 12%. The glass elements in the clay melt at peak temperature and solidify as the piece cools, increasing the strength of the finished pot.

Should the temperature of the kiln rise above the maximum temperature for that clay the whole pot can melt and make an expensive mess of the kiln shelf.

If a pot has a crack the piece cannot vibrate adequately and so it will not give a clear ‘ping’ and the sound will be dull and flat.

Welcome to my blog on bonsai ceramics.

I live on the Central Coast of New South Wales, Australia.

 I began pottery about 10 years ago and was quickly became addicted to clay. I had been enjoying bonsai since the 1990’s and making bonsai pots was a natural progression.

I love free form pots but also love refined classic pots. I have a gas kiln and fire up to Cone 10     ( 1300 degrees C), and I am still experimenting with textures, reduction firing, oxides washes and the application of multiple glazes on the one pot.

It is a joy to have a bonsai artist show me their tree in a pot I made, as well as seeing my trees and pots on my benches at home. 

Throughout the posts on this site my aim is to educate bonsai artists about age old craft of making bonsai pots.

Denise Allen

bonsaiceramics@yahoo.com

Copper Red Glazes

Copper Red Glazes are one of my favourite glazes. The earliest known copper-red glaze occurs on wares made in Shanxi during the Tang dynasty (AD 618 to AD 907). The secret of to how to make these lovely reds was then lost for several hundred years and there is a story about a royal potter taking the secret with him to the grave leaving a disgruntled Emperor.

Pot by Denise Allen 9 x 9 x 4 cm

Copper is one of the oxides that provide colour to glazes. Copper provides a green to turquoise colour to glazes in an electric kiln but can turn a wonderful red in a reducing atmosphere found in a gas or a woodfired kiln. This wonderful red is not guaranteed as copper is quite volatile and the red colour develops as it is cooling, after the pot has reached Cone 10 (approx. 1300 C). If the timing of creating the reduction atmosphere in the kiln is too late then you can end up with cream or pale grey coloured pot with perhaps a smudge of red.

The other complication is that the atmosphere in the kiln is uneven and some areas will have better reduction than others and some areas will be hotter because the air flows around and over the shelves. The best temperature to start reductions seems to be around 800 C. Copper red glazes nearly always “break” on a rim or texture and become cream or pale grey. There are several types of Copper Red Glazes, Ox-Blood or “Sang de Boef”,  Flambe and Peach Bloom.  Even the names are evocative.

Below are two tiles with identical glazes, the left one fired in a reduction atmosphere in a gas kiln and the right in an electric kiln.

from Greg Daly – Developing Glazes, Bloomsbury, 2013

Electric kilns usually have a programme that controls the firing (the rate of the rise of temperature in the firing eg rise from 800 to 1200 degrees Celcius at a rate of 100 degrees an hour) so they are switched on and then you can walk away and come back to find the firing cycle completed. A gas kilns like I use, needs constant monitoring of the temperature over the 10 – 12 hour firing cycle and the gas is increased or decreased manually as required along with the opening or closing of the damper and adjusting air vents to control the atmosphere in the kiln.

So what is reduction firing? In an oxidising atmosphere (ie. electric kiln) there is plenty of oxygen across the firing cycle but in a reduction firing the unburnt gas is starved of oxygen ( by closing down the damper and the air intake valves) so it seeks to take oxygen from materials in the glaze thus altering their composition. It is quite fun to open the spy hole in the kiln door and seem the characteristic flame associated with reduction, shooting out the spy hole. There is also a particular smell and flame noise to alert you that the kiln is in reduction.

This pot shows just a blush of copper red indicating that I did not get the reduction atmosphere right for greater colour. However, I like this effect.

I have recenty had the pleasure of spending 2 days learning from Pat Kenndy. He makes such clean lined, earthy pots with such attention to  detail. Pat  has  a wealth of knowledge and array of clever tricks and pratical approaches to ceramics that he generously shared. Now practice, practice, practice!