My Favourite Thing -A Hand Lens

Pottery hobby or interest in Minerals or Plants?

Some people just love gadgets particularly the new electronic ones. However here is one that has stood the test of time. Its just a hand lens- a magnifier you might say-but such a valuable piece of hobby kit.

Whether you want to look in detail at that pottery fault, or check there isn't a crack before you fire a piece this hand lens will save you so much time and money. Other interests such as looking at crystal minerals and fossils up close are also catered for.

Perfect Present For a Potter

A hand lens that lets you see the problem.

So often when you are learning to make pottery the results are not as you would expect or wish for. The piece has a small fault., a large speck or change in colour, just where you can see it! Use this neat little hand lens to see what really caused the fault.

Make sure that next time that perfect piece you made for a friend really turns out that way. Perfect

In this case it was possible to identify that the colour has crawled back leaving an area without colour.

Lenses or Loupe

The Same Thing!

Whether you are buying a magnifier for your pottery hobby or for studying the detail on jewellery, different names are often used. For example lenses used for looking at jewellery are often called loupe. In real terms they are all a magnifying lens.
However, the quality of lens determines the quality of image. So choose the best you can afford for something you will keep for a lifetime!

More information on pottery and ceramics can be found at my website http://www.pottersfriend.co.uk Go now to sign up for my free newsletter.

How to properly fire your pottery

The firing of pottery goes back over 3000 years. In those days wood fires were the source of energy. However today so many other forms of energy are available including wood, coal, gas, electric, oil, microwave, wind, solar to name just a few.

For the small pottery craft studio or hobbyist however the most common means of firing pottery are by the use of gas or electric kilns. Generally electric firing is a clean means of firing whilst gas is thought to be more energy efficient. Let us consider the basics of each:

How does electric firing work?

The principle of electric firing is relatively simple. Imagine a box or cylinder kiln fitted with heating elements like your domestic oven. However, instead of heating to 300C it is capable of heating to 1300C.

Operation of the kiln starts when electricity is applied to the elements ( resistive load ) causing them to heat rapidly. The heat is transmitted to the pottery mainly by radiation. Special insulating bricks, called refractories, line the floor,walls and roof of the chamber to prevent the heat generated from escaping. An electronic box, called a temperature controller,linked to a thermocouple (special wire) inside the kiln, measures the temperature constantly in the chamber of the kiln and then controls how much electricity is supplied to the elements. This in turn controls the rate of increase or decrease in temperature within the kiln.

How does gas firing work?

The principle of gas firing is also relatively simple.. Imagine this time that the box is kiln fitted with a burner or burners, a little like your domestic gas oven. However, instead of heating to 300C it is capable of heating to 1300C and because of the large amount of gas used the kiln needs a chimney or flue.

Operation of the kiln starts when the gas is turned on and burner lit. The heat is transmitted to the pottery mainly by convection and radiation causing it to heat also. Special insulating bricks, called refractories, line the floor,walls and roof of the chamber to prevent the heat generated from escaping. A vent in the roof or rear of the kiln leading to a chimney allows combustion gases to be removed from the kiln. An electronic box, called a temperature controller,linked to a thermocouple (special wire) inside the kiln, measures the temperature constantly in the chamber of the kiln and then controls how much gas is supplied to the burner. This in turn controls the rate of increase or decrease in temperature within the kiln.

How do I fire the kiln properly?

For both electric and gas firing the two critical factors which affect the success of your firing are peak temperature and uniformity of temperature within the kiln. Peak temperature can easily be measured using the thermocouple fixed inside the kiln. Uniformity could also be measured using a series of such thermocouples located in different sections of the kiln. However this would be an expensive option for smaller kilns.The use of Bullers Rings or pyrometric cones to confirm the consistency and uniformity of firing at different locations within the kiln is more cost effective.These ceramic cones or rings can be placed next to your ware and will experience exactly the same amount of heat as your item. By measuring the values of the rings or cones after each firing you can be assured the firing is consistent from one firing to the next.

But what is the peak temperature?

The peak temperature on firing biscuit or glaze is the maximum temperature reached on firing. This value depends on the type and composition of the clay body and glaze used. As there are literally thousands of body and glaze compositions,suppliers of body and glaze products therefore recommend a peak temperature or ring or cone value equivalent suitable for optimum firing their products. It is advisable to follow their recommendation.

But what if the temperature isn't consistent?

Quite often there is a spread of temperature within the kiln. It is quite common to have a 20 C spread in temperature between the bottom and top of even small kilns. The reason for this is dependent on a number of factors. Rate of firing, kiln design and condition (age and condition of brickwork),element condition and age (for electric kilns) gas/air ratios and flame (for gas kilns) and ventilation aspects of the kiln are all key factors.

For uniform firing of kilns a slower temperature rise to peak temperature and a soak at peak temperature will reduce variation of temperature within the kiln. Indications of non uniform firing of glazed pottery are variation of gloss, colour and or bubble between similar fired pieces.

Summary

Peak temperature and uniformity of temperature are the key factors determining the success of your pottery firing. Peak temperature is measured using a thermocouple connected to the digital display of a temperature controller. Uniformity is measured using Bullers rings or Cones placed in the kiln next to the ware. The heating rate has an affect on both and should be relatively slow for best results. A cycle of more than 12 hours cold to cold is quite common. However you should be guided by the body or glaze supplier as they know the optimum cycles for their products.

More information on firing of pottery and ceramics,including Bullers rings and Orton cones, can be found at my website http://www.pottersfriend.co.uk Go now to sign up for my free newsletter.

Raku Pottery

What is Raku Pottery?

Raku is a pottery technique that originated in Japan in the 16th century. The technique involves a specific rapid firing process at a relatively low temperature but the whole process can be much more complex.
The technique has developed and there is now the traditional Raku and a western version of Raku. The results of this type of process are very unique from other firing techniques, and some fantastic metallic finishes, crazing and other decorative effects can be achieved.

The process in brief

In raku, the pottery pieces are loaded into a cold kiln (for multiple batches pieces are often preheated and loaded into a hot kiln). The kiln is heated rapidly with the ware reaching temperature in as short a cycle as 15-20 minutes.This contrasts with traditional firing cycles of 10 hours or more!

Glaze maturity is judged by the trained eye without the use of cones or measuring devices. Often this is when the glaze has begun to melt.
When the firing is considered to be complete the ware is immediately removed from the kiln while still red hot using tongs or another lifting device.This is the stage in the process where traditional and western raku differ in technique and treatment.

In the western version the ware is treated to a post firing reduction. To achieve this the hot ware is put into a container, usually a garbage can, with combustible material such as paper, sawdust, or leaves and allowed to smoke for a predetermined length of time (usually about 20 mins). The carbonaceous atmosphere (black sooty smoke) reacts and affects the glazes and clay and imparts unique effects and surfaces to the wares. Some of these effects are metallic and crackled glazes surfaces and black unglazed clay.

When the ware has cooled, it is washed with an abrasive cleaner to remove all residues of soot and ash. In this way it is possible to produce a unique piece of art pottery. Unfortunately because of this rapid process the ware is relatively fragile and the decoration not food safe. Pottery made in this way must only be used for decorative purposes.

The Process in detail

The Clay body

Raku bodies are designed so that they allow rapid heating and cooling without cracking. Most raku bodies (but not all) use a recipe that is high in grog (coarse material). A range of Raku bodies are available from suppliers to match your making and firing process.

A typical body recipe might include by parts weight:-

Clay 65-75%
Grog 25-35%
Talc 0-15%

The type and amounts of each component will depend on many factors. These include the clay making process, the fired body colour and texture required and the firing temperatures to be used.
One of the important questions you need to answer "will the glaze and body combination I intend to use give me the glaze finish I require?". For example many glazes used on Raku will readily craze if the correct glaze/body combination is not used. For some this might be an appealing effect. For other Raku makers it may not. So choose your clay body wisely.
The clay body for raku is typically bisque fired to cone 04 (1060C) prior to glazing. This gives it sufficient strength for handling during the glaze firing process, as well as some porosity to aid glaze application.
Most clay suppliers will help you choose an appropriate clay body. Biscuit firing of the clay piece is essential if the pottery is to avoid breakage on firing.

The Glaze

Next you can apply slip, apply glaze, or just leave the pot bare. Any glaze that will melt successfully during the raku fire could be used. The most important factor in identifying potential raku glazes is the temperature at which they mature, how you plan on using them, and what kind of effects you are looking for. Low fire glazes generally give the best results. Most raku is fired in the range of cone 010-06. (900-1000C) Note the surfaces of the pot that are not covered with glaze will however turn black from carbon from the post-firing reduction process.
For more info on Raku glazes see my separate article on Raku glazes.

Raku (glaze) firing

Northern Kilns Raku kiln

The pottery is placed in the kiln and rapidly heated to the point at which the glaze melts. Although a pyrometer is sometimes used to monitor how fast the temperature is rising, Raku artists usually watch the glaze to see when it is ready to be removed from the kiln. Depending on the glazes and technique used, this might be when it gets shiny, or when it bubbles and/or looks like orange peel. Transfer of the ware from kiln to the reduction chamber is important. Speedier transfers can give metallic lustres whilst slower transfer can give brighter colours.

Types of Raku kiln

For firing of multiple loads the temperature of the kiln must also be readily controllable. An electric kiln is perfectly suitable for raku although there are some special considerations that require careful attention. Cut off switches must be used to avoid any contact with live electrical current when the electric kiln is opened. Many types of gas and electric kilns can be used for raku. Kilns need to be located outdoors or in close proximity to the outdoors or have lots of ventilation. Access to the pots inside the kiln is critical so they can be easily removed. Raku kilns are therefore designed to provide this easy access to red hot pots.
More information on Raku firing can be found the Potters Friend website

The Reduction Process

The reduction chamber (garbage can) is prepared by adding the combustibles. There are many things that can be used, but newspaper, sawdust, or wood chips work well and is abundant. After switching off, the kiln is opened and hot piece removed from the kiln with tongs or a lifting device. Immediately it is placed in the reduction chamber (garbage can) where the heat sets the combustibles alight. The lid of the can is replaced and the chamber left for about 20 minutes.

After the pot has completely cooled, it is possible to use Ajax cleaner or similar to clean carbon off the glaze and "shine up" the piece. Reduction is a firing term that refers to a lack of oxygen in the combustion process. By sealing the can with the lid the can is deprived of oxygen. This lack of oxygen causes incomplete combustion of the paper or sawdust to take place and creates the metallic finish on the glaze.

Safety

The raku process is a dangerous process as you will be exposed to high heat, flames and smoke. Always wear fire protective clothing, fireproof gloves and protective eyewear. Do not open or insert tongs into an electric kiln without shutting off. The reduction process (container with combustibles) should take place outside of building due to flames and smoke. Do not apply spray glazes without a NIOSH approved respirator and other personal protective equipment. Although many glazes are Non Toxic and Non Hazardous when used according to manufacturer’s directions, this is not always the case. It is important to check! Raku glazed items are suitable only as decorative ware and are not recommended for contact with foodstuffs.

How to Remove Tea and coffee stains from your Ceramic Mugs

It is a common occurrence for mugs used everyday to become badly stained by tea or coffee. The brown stains are unsightly and difficult to remove. Sometimes they become so bad that scrubbing in soapy water does not remove them. Clearly coffee and tea are strong staining agents!

"But what causes the staining?"
It is a combination of hard water, chemicals called tannins, and microscopically rough glaze surfaces. These conditions allow the stains to strongly adhere to the glaze surface. The staining is usually worse in the base of the mug or where there is a sharp corner to hold on to the stain.

"So how do I clean stained mugs?"
Various cleaning methods have been tried and tested including vinegar, bleach, baking soda and washing soda. All have been proven to be successful and are widely used. However Bar Keepers Friend and tooth cleaning powders are less likely to give long lasting damage to the glaze surface. Cleaning with other chemical compounds and scourers whilst successful can create damage to the glaze surface which will make them more susceptible to future staining. So please take special care with your best china or dinnerware and treat them gently for their long term protection.

"So how do I stop staining?"
Manufacturers of the top brands of dinnerware and mugs design their products to resist staining and damage in use. In essence this is done by ensuring the glaze is chemically and physically durable. However,with the widespread manufacture of mugs in many parts of the globe including Asia, it is not easy to know the source and quality of the ceramic mugs we buy.It is almost impossible to assess how resistant mugs will be to staining by visual inspection. Smooth glaze surfaces with few corners will reduce the potential for the stain to adhere strongly to the surface. However, over time the glaze surface may be attacked by dish-washing agents. leading to the dreaded staining.Cleaning the pottery soon after use with warm soapy water will certainly reduce the possibility of staining.

"Summary"

To reduce tea and coffee staining of your mugs, clean them as soon as possible after use in warm soapy water. Use quality mugs from top brands as they will have designed the mugs to be chemically and physically durable. Avoid harsh detergents for cleaning as this can damage the glaze surface and lead to greater staining. Bar Keepers Friend and tooth cleaning powders are preferred cleaners and less likely to create long term damage. If all else fails buy new quality mugs but recycle the old ones. Pottery is a truly 100% recyclable product!

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend. Go now to sign up for my free newsletter.

Is Your Pottery Microwave Safe?

Microwave ovens are now a common feature in most kitchens. Their use both to heat and cook food means a wide range of ceramics is being used in microwave ovens. However the question remains….Is all pottery microwave safe?

Technically this question is a little more complex than it appears. There are general rules such as:-
“do not use ceramics decorated with gold, silver or precious metal in microwave ovens”

 Indeed ceramic with any metal should not be used in a microwave oven. However additional rules depend on recognising which type of ceramic is being used. For the consumer this is not an easy task.

All ceramics are not the same!

Ceramics are now made all over the world and it is not always easy to establish their source or quality or type. However ceramics that have been fired to a high temperature and are non porous are generally considered to be acceptable for use in microwave ovens. These ceramics include stoneware, porcelain, bone china, and vitreous cookware. Other ceramics which have not been fired to a high temperature have to be used with caution. For these type of product to be used safely there is a need to ensure that they are both dry and have not absorbed water. This can happen in many circumstances particularly during washing or in storage in the fridge. These products need extensive safety instructions to be used safely.

Top brands help the consumer by labelling their products as ‘microwave safe’. Less well known products cannot give this assurance and often do not label at all. So buyer beware and if in doubt do not use in a microwave. Look out for BS or CE standards as a sign of meeting a quality standard.

In summary, most high fired ceramics are suitable for microwave oven use. Lookout for the labelling used by branded products to give reassurance on acceptability for microwave oven use. If in doubt “keep out” is a good motto to remember!

Why not test your pottery knowledge with my quick pottery quiz

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend

Go now to sign up for my free newsletter.