A More In Depth Look At Mugs

There are over 6 million branded promotional mugs sold in the UK each year to organisations of all sizes from small clubs, start-up businesses through to large corporates. They are one of the top selling promotional items of all time and even as new innovative product lines arrive on a yearly basis they still remain at the top of the list for advertisers.

Below we have set out in great detail all the information you could possibly wish to know about when it comes to mugs and we hope that you find it highly informative.

A mug is a robust type of cup which is commonly used for drinking hot beverages, for example tea, coffee, hot chocolate, Bovril or soup. Mugs by their very nature, feature handles and more often than not have a capacity to hold a larger amount of liquid when compared to other types of cups such as tea cups. In the majority of cases a mug will hold approximately 12 fluid ouncs (350ml) of fluid, twice as much a tea cup. Mugs are generally thought of as a less formal drinking vessel and as such are rarely used in formal settings, where a teacup or coffee cup would be the preferred choice.

Historically, mugs were originally carved from wood commonly known as a KÃ¥sa, bone or clay, however the majority of modern mugs are made from ceramic based materials such as earthenware, porcelain, fine bone china or stoneware. In relation to travel mugs these are usually manufactured with a double walled insulation in either plastic or metal where break resistance is a priority and this also provides for a lighter weight construction perfect for those on the move.

For decades the majority of mugs used for promotional purposes were manufactured by UK based companies and predominantly in Staffordshire, otherwise known as the Potteries, even having one of its football teams named after the success of the area, Stoke City Football Club - The Potters! However, in recent years this vibrant manufacturing area has suffered a decline from increased competition in China and these days the majority of mugs are manufactured abroad and brought in by wholesalers who then simply apply the relevant print process and sell them on.

At one point even The Prince of Wales led a charity to try and step in to save one of the last working Victorian potteries from closure. For more than 100 years artisans have been turning out sought-after Burleigh porcelain pottery from the kilns of Middleport. But like other potteries in North Staffordshire, which was once the centre of the world’s ceramic industry, it has been hit by the economic downturn and competition from overseas. However there is a museum on the original site which can be visited at:

Middleport Pottery
Port Street, Stoke-on-Trent
Staffordshire ST6 3PE
Tel: 01782 575565.
Click here for directions

This trend may well change in the coming months and years as the Council of Europe in Brussels (1000 City of Brussels, Belgium) has now imposed a hefty import tax duty on earthenware products imported direct from China, thus making them less competitive and could hail the revival of the pottery industry here in the UK.

A Look At Mug Construction

Earthenware Mugs

Earthenware is a form of pottery which can be fired a low temperatures, thus ensuring that the mineral components do not vitrify, or simply turn into glass. This unique feature ensures that it is both porous and opaque, and will always retain its rich clay colour. Earthenware has been made for well over 9,000 years and remains a highly popular choice for mug manufacturers.

The blend of materials in earthenware varies, depending on the region, but it generally includes minerals such as quartz and feldspar, along with ball clay, a very plastic natural clay, and kaolin, or china clay, a more mineralized form. These materials are ground so that they have an even texture, and they are worked on a wheel or by hand into the desired shape. Earthenware is first bisque fired and then fired again at a temperature which may be higher or lower, again depending on the style.

Earthenware features:

• Not impervious to water (cannot hold water)
• Chips easily
• Color: white
• Feel: chalky
• Look: rough white
• Can only use when decorated
• Cannot withstand high/low temp.
• Not oven safe
• Suited for decorative or promotional use
• Painting bisque: Cannot correct mistakes
• Glazes adheres to bisque: bisque is porous
• Paint or glaze the bottom of the piece and stilt on shelf

Examples of earthenware mugs

There are a raft of different styles of earthenware mugs which can be custom printed with corporate logos and marketing messages and are predominantly used for promotional purposes, but below are just a few of the most popular designs which are available on the market.

Porcelain Mugs

Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between 1,200 °C (2,192 °F) and 1,400 °C (2,552 °F). The toughness, strength, and translucence of porcelain arise mainly from the formation of glass and the mineral mullite within the fired body at these high temperatures.

Porcelain derives its present name from old Italian porcellana (cowrie shell) because of its resemblance to the translucent surface of the shell. Porcelain can informally be referred to as "china" or "fine china" in some English-speaking countries, as China was the birthplace of porcelain making. Properties associated with porcelain include low permeability and elasticity; considerable strength, hardness, toughness, whiteness, translucency and resonance; and a high resistance to chemical attack and thermal shock.

For the purposes of trade, the Combined Nomenclature of the European Communities defines porcelain as being "completely vitrified, hard, impermeable (even before glazing), white or artificially coloured, translucent (except when of considerable thickness), and resonant." However, the term porcelain lacks a universal definition and has "been applied in a very unsystematic fashion to substances of diverse kinds which have only certain surface-qualities in common" (Burton 1906).

Bone China Mugs

Bone china is made of 50% bone ash, & 25% each of china clay and china stone. It is combined with water to make a slurry, which is then fashioned into cups, saucers, plates and so on. The pieces must be fired in a kiln at high degrees of heat (1100c to 1250c) for the china to become hard and strong. It is the bone (usually animal bone, cleaned of all meat and glue) that gives the china its transparent whiteness.

The ingredients of bone china are china clay, china stone, silica, alumina, alkalies, lime and bone ash. It is fired at high of 2,300 to 2,500 degrees Fahrenheit. In the best grades of English bone china, 50% or more of the body consists of refined bone ash prepared from specially selected animal bones which are reduced to a fine powder by heat. The bone is mixed with the finest china clays and highest quality Cornish stone. Specially skilled workers are needed in the manufacture of bone china and the best and most highly skilled in the world are found in the pottery district of England.

Examples of china mugs

China mugs are often used for special occasions such as company anniversaries or used to be gifted to high achieving employees or discerning clients, below are a few examples:

Stoneware Mugs

Stoneware is a stronger clay that is fired to a high temperature than traditional earthenware mugs (about 2185 degrees Fahrenheit) and becomes vitreous. It can then be left undecorated or decorated with coloured glazes with an optional clear glaze coating and re-fired.

Stoneware is a clay that when fired to maturity becomes a sturdy, chip resistant material suitable for using in cooking, baking, storing liquids, as serving dishes and even to use in the garden. These types of mugs are exceptional durable and therefore provide for added longevity.

Stoneware features:

• Impervious to water (water tight)
• Chip resistant
• Color: Buff or terra cotta
• Feel: textured
• Look: like pottery
• Looks great undecorated or decorated
• Can withstand high/low temp
• Oven safe
• Suited for household use
• Painting bisque: Wipe off mistakes. 
• Glazes flow: bisque is not porous
• Firing: Leave the bottom of the piece dry and place on shelf

Dye Sublimation Photo Mugs

If you are looking for full colour photo graphic quality promotional mugs then dye sublimation mugs are the ideal product.

These unique mugs are printed utilising a unique process which allows for the reproduction of photographs and exceptionally complex graphics to create a stunning finish.

Great photography can do many things; evoke emotions, generate ideas and sell products. With dye sublimation printing it's also possible to get great photographic images on mugs. Dye sublimation is the perfect method of getting designs onto ceramic mugs, due to the unique printing process which can achieve colours and designs not normally possible with standard ceramic printing methods.

These mugs are available in a whole host of different styles to suit various marketing projects and are exceptionally popular amongst marketing professionals. There are generally two options with these, a low cost budget version which will with stand up to around 72 dishwashers, however a higher quality version is available which features a unique Duraglaze coating.

This exclusive coating patent protected coating means that we are the only company in Europe  that can offer the only 100% guaranteed dishwasher proof Photo Mugs, independently tested in Europe to in excess of over 2000 washes!

Examples of dye sublimation mugs

These promo mugs are available in a variety of different designs to suit all tastes and below are just a few examples of some of the options available.

Travel Mugs

 Travel mugs (introduced in the 1980s) generally employ thermal insulation properties for transporting hot or cold liquids. Similar to a vacuum flask, a travel mug is usually well-insulated and completely enclosed to prevent spillage,  but will generally have an opening in the cover through which the contents can be consumed during transportation without spillage. Mugs with inner and outer wall but not vacuum treated are generally called double wall mugs. Usually stainless steel will be used for the inner wall while outer wall can be stainless steel, plastic or even embed with other materials.

Much of the mug design aims at thermal insulation: the thick walls of a mug, as compared to the thinner walls of teacups, insulate the beverage to prevent it from cooling or warming quickly. The mug bottom is often not flat, but either concave or has an extra rim, to reduce the thermal contact with the surface on which a mug is placed. These features often leave a characteristic O-shaped stain on the surface. Finally, the handle of a mug keeps the hand away from the hot sides of a mug. The small cross section of the handle reduces heat flow between the liquid and the hand. For the same reason of thermal insulation, mugs are usually made of materials with low thermal conductivity, such as earthenware, bone china, porcelain or glass.

Travel Mug features:

• Insulated, keeping beverages hot and hands comfortable
• Double-insulated travel mugs offer better performance
• Plastic versions come in a variety of colour options
• Stainless steel travel mugs are easier to clean and keep beverages hot for a longer period of time
• Made to fit easily within vehicle cup holders
• Perfect for Competitive sports, Fitness, Hiking, Climbing, Skiing/Snowboarding, Boating and Hunting to name just a few

Examples of travel mugs

Travel mugs come in a whole host of designs and price points to suit all budgets and tastes, below are just a few examples.

Plastic Mugs

Some of the benefits of using plastic mugs for promotion is that they are lightweight, durable and break resistant which makes them perfect for schools and the younger generation where accidents can happen and as such there is little danger should they get dropped. They also come in a whole host of solid and translucent colours that can give a promotion a fun, modern angle.

A plastic mug cannot, of course, exist without plastic. The key to plastic is its molecular structure, which is fused together by covalent chemical bonds. Because of the synthetic materials plastic is composed of and its elasticity, it is ideal for the mass manufacturing of products like mugs

There are generally four ways to mould plastic into a desired shape: Extrusion, blow extrusion, injection moulding and blow moulding. For the manufacturing of plastic mugs, the most effective method is injection moulding, wherein hot plastic is injected into a mould (in this case, one that is in the shape of a mug) and the shape is then mass produced for maximum output.

Plastic Mug features:

• Perfect for school promotions where accidents can happen
• Exceptionally durable for added longevity of your message
• Available in a host of colour options including unique glitter finishes

Examples of plastic mugs

Plastic mugs are available in a massive range of shapes, designs, colours and even some which are manufactured from recycled plastics for the environmentally conscious amongst us, below are just a few examples to provide you with some inspiration.

Specialty Mugs

The world of promotional mugs has moved on significantly over the past decade and the options available now are immense providing marketers with a whole raft of new ways to ensure that their advertising campaigns can stand out from the myriad of standard more traditional options. Below are a few options to get your creative juices flowing.

Heat Change WOW Mugs

These unique mugs are called WOW mugs for excellent reason. These clever and unique mugs are coated with a special heat sensitive layer over the top of your desired image, which becomes transparent when hot liquid is added thus magically revealing your design. As the mug cools down, your image slowly disappears again, making for an eye-catching and entertaining corporate gift.

These high quality heat change mugs offer the best quality print finish of any ceramic product and can also be screen over-printed with a permanently visible message to convey the brand or advertising message when the mug is cold. These mugs are inspected no less than 7 times during the production process to ensure clients receive the best possible quality.

These mugs are great for promotions or simply reinforcing business relationships. Maybe your staff and clients are 'the hottest team in the business? Why not reward them with their very own special 'heat change' slogans to leap out at them every time they fill their mug. The organic colour is cured into the spray coating, unlike ceramic colour, which is fired into the glaze. Hence the need to avoid dishwasher or microwave use.

Due to the nature of the specialist coating these mugs are not suitable for use in microwaves and are not dishwasher safe. We therefore recommend hand washing only with mild detergents in moderate temperatures to ensure these mugs remain as bright and vivid as the day they arrived.

Examples of Specialty Mugs

Mug Printing Techniques

There are a whole host of different print methods available on printed mugs and the best option will generally depend on the design or logo to which one is best. Below we look at the various printing techniques available.

Direct Screen Printing

Ideal for: earthenware; cost effective in both large and small runs and can be used on high quality bone china also.

Not for: complex half-tones

An image, usually up to four line colours, is printed within a given area directly onto the rotating body of the mug - check out this video of direct screen printed mugs. The method is used when:

• the design is simple
• there are no complex half-tones although some half-tones can be directly printed
• the registration is not over demanding
• the design is not required to be larger than the print area

There are limitations with this process and it is primarily used for the low budget side of the earthenware market. A direct printer can print several thousand items a day, in contrast to the slower, hand decorated transfer process.

Transfer Printing

Ideal for: earthenware, stoneware and bone china; allows for more complex art tones; critical registration and full colour designs; printing on the full surface area of the item; highly-shaped items

The design is silk-screen or litho printed onto special paper, then cover-coated. The resulting print is then treated as a water slide transfer and applied by hand to the ware. The process is slower and more labour intensive than direct screen printing but is more flexible with colours able to be built up with precision and control. Printing is possible on the full size of a mug, inside on the base, as a back stamp, or just inside the rim. Check out this video on transfer printed mugs.

Transfer or litho printing, as opposed to direct silk-screen, is used when:

• larger than average print is needed
• working with bone china
• printing in four-colour process
• cases where more than four line colours, very fine detail or tight registration is required.

Digital Printing

Ideal for: full colour photographic images or cartoon images without bright primary colours; both earthenware and bone china; small print runs.

Not for: any images with strong primary colours or coloured glazed ceramics.

The image is imprinted directly on to a unique piece of coated paper in vibrant full process colour very similar to that used in a high end office digital printer. The paper is then cover-coated creating a what is called a water slide transfer which then in turn is hand applied to the surface of the mug. Similarly to full colour printing, there can be some limitations on the vibrancy of certain colours such as reds or pinks, however with citrus colours or shades the end results are superb. The inks used in this process are naturally based, and as such specific pantone colour matching is not achievable, however for photographic reproduction this technique is excellent. Watch this video for more information on digitally printed mugs.

Laser Etching

Laser Etching results in an embossed image carved into the glaze of an earthenware beaker, exposing the white clay biscuit of the mug. This gives the product a tactile appeal as well as visual.

After the etching process, the ware is fired at a very high temperature to render the biscuit non-porous, preventing the absorption of contaminants through the exposed surface.

Special Additional Print Options On Mugs

Not only can mugs be printed as per the options outlined above, there are however a range of other printing options to ensure that your promotional mugs really do stand out from the crowd:

Eyeline Printing

It's a simple idea but highly effective and economical - your message, phone number or website address printed right on the eyeline, up close and personal, where it can't be missed. So just imagine every time the recipient of your promotional mugs take a sip of their favourite beverage they will be reminded yet again of your message. This makes for amazing subliminal additional advertising.

Handle Printing

It's a totally different way to brand a mug - your website address, phone number or branding on the handle which adds just that little more sophistication to your marketing efforts and will certainly ensure that your mug stands out from the crowd.

Backstamps

Backstamps are prints on the outside base of the mug. They can be used to put individual numbers on a limited edition run or adding words, such as 'bone china', to reinforce the value of the item.

Gilding

The addition of a coloured band to the rim of the mug, particularly a colour at the heart of the surface decoration, imparts style to what may seem an ordinary design.

AntiBug coatings

Mugs can now be treated with a  patent protected, fully certified AntiBug treatment to keep germs at bay. The specialist surface starts to fight germs as soon as they come into contact and has been clinically proven to kill 99.9% of harmful bacteria from 50 common organisms such as MRSA, E.coli and salmonella and will continue to fight for the lifetime of the product.

Anti-Bug mugs are ideal for infection control initiatives, domestic cleaning brands, doctors surgeries, hospitals, food preparation areas, call centres and offices, nursing homes, prisons, residential homes and hotels.

When printing on to mugs or ceramics the colours in the glazes can often be affected by the clay, slips, stains or even the underglaze beneath them. The majority of ceramic colours, however, are a direct result of the metallic oxides that are dispersed in the fabric of the actual glaze itself. Subject to the circumstances, these colourants can display a variety of different results.

The Three Major Factors Which Effect Glaze Colour

There are a variety of different variables that can affect the a glaze's end colour. These factors in the main fall within a group of three key areas.

• The makeup or composition of the actual glaze. This is not primarily relating to the colourants in the glaze itself, but also take into account the myriad of other glaze materials that ultimately interact and effect the colourant combination of colourants.
• A key factor is the temperature to which the glaze is fired. Specific colourants are highly volotile and often dissipate into the kilns atmosphere if fired to an extreme temperature.
• Also key is the kilns atmosphere throughout the firing process and in some cases even during the cooling process.

Chromium Oxide

Chrome oxide can create a whole host of different colour finishes: yellow, pink, brown red and inparticular green. Chrome is an exceptionally volatile element at cone 6 and can often jump from pot to pot creating streaks and strange smoky effects.

• Chrome-red: this requires a lead glaze which is fired at cone 08 or less. Exceptionally toxic, and certainly not for functional earthenware.
• Chrome-yellow: this needs a lead-soda glaze which is fired at cone 08 or lower, if not it will begin to turn green. Exceptionally toxic, and certainly not for functional earthenware.
• Chrome and zinc yield brown.
• Chrome plus tin yields pink, grayed pink, and warm browns. The Colour depends on proportions of these oxides in glaze and in relation to each other.
• Small amounts of chrome plus cobalt can yield teals at cone 9 and higher when fired in reduction. Magnesia glazes aid in producing nice colours.

Manganese Dioxide

Manganese  is usually introduced into glazes as manganese carbonate. Black manganese dioxide is more often used in slips and clay bodies, where its coarseness yields spots and splotches. Manganese, when compared to cobalt or copper, is a fairly weak colorant. It is toxic; handle with caution, using all safety precautions.

• In high-alkaline glazes, manganese yields rich blue-purple or plum.
• At cone 6 and above, manganese produces brown.
• In lead glazes, manganses yields soft purple tinged with brown. Extremely toxic; not for functional ware.

Cobalt Oxide and Cobalt Carbonate

Cobalt is an extremely powerful colorant that almost always produces an intense blue. Cobalt carbonate tends to be used more by potters because it has a finer particle size and is less intense.

• In glazes with a high magnesia content, very small amounts of cobalt can give a range from pink through blue violet.
• Magnesia and cobalt in glazes fired at cone 9 or higher can yield blue mottled with red, pink, and purple. Very hard to control and duplicate due to the narrow temperature and atmospheric range.
• Cobalt and rutile can produce in mottled and streaked effects.
• Cobalt with manganese and iron will yield an intense black.

Iron Oxides in Clay

Not many manufacturers would challenge iron's place as one of the most important of the ceramic colourants. Iron's natural presence in most clay bodies produces clay colours which range from light grey to deepest brown. Under clear glazes, iron-containing clay bodies can show a very similar range of colours.

Iron-containing clay bodies that have been fired but are not mature, such as bisqueware, often are a salmon or yellowish pink colour. If a pot is glazed with a lower-temperature glaze and fired below the clay body's maturity temperature, a salmon, ocher or reddish brown colour will show through.

Iron Oxide Types

Most iron used in glazes is introduced as red iron oxide (ferric oxide, Fe₂O₃). Yellow iron oxide is another form of ferric oxide; although its raw colour is different, it is chemically identical to and acts the same as red iron oxide. Black iron oxide (ferrous oxide, Fe₃O₄) is courser and generally not used. Crocus martis is an impure iron oxide which can be used to produce speckled, rough, or spotty effects.

Iron Oxide in Glazes

Generally speaking, iron produces warm colours which range from light tan and straw to deep, rich browns.

• High-fire glazes containing bone ash and iron can yield persimmon oranges and reds.
• Iron and tin in high-fire glazes result in a mottled cream color, breaking to red-brown in thin areas.
• Iron fluxes in reduction atmospheres. It is less active and can sometimes even act as a refractory in oxidation atmospheres.
• Iron in high-fire reduction can yield lovely, delicate iron-blue and celadon green.
• High-fire, high-iron content glazes fired in reduction will yield glossy dark brown or brownish black. In thin areas, the iron may reoxidize during cooling. Reoxidization will result in those areas turning red or gaining red highlights.

Copper Oxide and Copper Carbonate

Copper is a strong flux which can make a glaze more glossy. At cone 8 and above, copper is volatile and can jump from pot to pot. Copper generally gives green in oxidation and red in reduction. Copper oxide is more intense than copper carbonate, as it contains more copper by weight.

• In alkaline glazes, copper will produce turquoise.
• Copper yields a lovely range of greens in lead glazes. Copper increases lead's solubility. Toxic; not for functional ware.
• Copper in barium high-fired glazes produce intense blue and blue-green in both oxidation and reduction. Toxic; not for functional ware.
• Copper in low-fire raku glazes can yield metallic copper. Over time, however, the glaze will oxidize to green.

Nickel Oxide

Nickel oxide, when used by itself, gives notoriously unpredictable results. It can be used to produce quiet grays and browns, but nickel is almost always used to modify and tone-down the colors produced by other colorants.

Rutile

Rutile is an impure titanium ore containing some iron and other materials. It is a very interesting colorant which is generally tan in oxidation and gray in reduction. Rutile encourages crystal growth in mid-range and high-fire glazes. It is well known for creating lovely streaky and mottled effects.

• In boron-containing glazes rutile produces pronounced streaks or spots, especially in glazes containing other colorants.
• In fluid glazes, rutile encourages opalescent blues.
• Rutile increases opacity.

Various Other Colorants

Other colorants that are used less often in the manufacturing of mugs include:

• Antimony: used for yellow in low-fire glazes.
• Cadmium and Selenium: very similar, producing bright reds. Both burn out extremely easily. Toxic; not for functional ware.
• Gold: gives a range of red, pink, and purple.
• Ilmenite: as a colorant, very similar to black iron oxide.
• Iron Chromate: produces shades of brown, grey and black. Iron chromate plus tin may produce a pink or reddish brown; if applied with a brush, can yield black hazed or haloed by pink. Toxic; handle with care.
• Platinum: provides grey.
• Silver and Bismuth: used in luster overglazes.
• Uranium Oxide: gives coral, yellow and red colours. Note: even fired into a glaze, uranium remains radioactive. Toxic; handle with care.

Mugs The Drying & Firing Process

It is imperative that manufacturers have an in depth understanding of the process that clay goes through during the drying and firing process to avoid firing defects such as cracking, breaking and even exploding or simple glaze defects. Key to this is ensuring that the clay is allowed to dry in a slow and uniformed manner.

Shrinking

Throughout both the drying and firing process, clay shrinks. Subject to the type of clay, it will shrink at different rates from as little as 4% through to as much as 15%. Amazingly, even a one percentage shrinkage can make the difference between first grade finished product or a second grade product with defects. The standard clay used in promotional mugs is generally Baldwin 192 which is designed to shrink 11% for every cone 6 firing temperature. Industry tests have proven that around 5% shrink during the drying process, 1/2% during bisque firing (cone 06) and 5.5% during the final glazing process (cone 6). This amounts to a total of 11% shrinkage across the manufacturing process.

It has also been proved that subject to the age of the clay there can be a variance of 1-2% from any particular batch. It is highly advised to allow for a variance of +/- 3% when order promotional mugs.

Why is it important to let clay work dry slow?

Be aware that the larger to piece is, the more movement of shrinkage will take place. For a example, if clay shrinks 5% during drying, a piece of clay that is 5" long, will shrink 1/4" and a piece that is 20" long, will shrink 1" during the drying. This is why it is beneficial to know that the larger the piece is, the more important it becomes to let it dry evenly (slow drying will yield even drying). See next chapter about Air Drying. If one part dries faster than the other part, the dryer part will shrink more on wetter parts which will create stresses on the clay, thus, cracking could occur.

Shrinkage volume reduction

If the total shrinkage of clay is specified to be 11%, it will yield a 30% reduction in volume (cubic inches) which is important to account for when making mugs of any sort.

Air Drying

Wet clay contains a large amount of water, a minimum of 25% water. When clay starts to dry, water evaporates from it. As this happens, the particles of clay are drawn closer together resulting in shrinkage. Many problems with clay are formed by uneven rates of drying, which create stresses in the clay. Sometimes these stress show up right away as cracks or warpage, other times not until during or even after firing. So it is important to ensure drying is even. This is done by ensuring uniform thicknesses throughout the piece, drying slowly, and even slowing down the drying of certain parts.

Clays which have very fine particle sizes will shrink more than clays with larger particle sizes. Porcelain clay has very fine particle sizes which makes it very plastic and also shrinks the most. These bodies have the most strength in the dry state. Groggy clays such as sculpture bodies shrink the least. (Grog is clay which as already been fired and then ground to various particle sizes.) These bodies shrink less because they have lower water content to start with, and also provide channels through which moisture can escape toward the surface. These are called "open bodies".

When the water has evaporated form between the clay particles, and all the remaining clay particles are in contact, drying shrinkage is complete. This is called the leather hard stage. The particles themselves are still damp, but their drying will not cause any additional shrinkage.

Expediting Drying

If mugs are damp or slightly wet, sometimes it is ok to expedite the drying by using a fan, warm kiln room or candling in the kiln. Generally, when candling in a computerised kiln going from ambient room temperature to 180 degree F, the temperature rise is slow and it takes approximately 2 hours to get there allowing the last minimal shrinkage and evaporation of the remaining water to occur gradually and slowly.

If damp mugs are placed in the kiln room when the room is not too hot and the kiln room temperature rise is slow, it will have a similar beneficial effect as candling. However, if you place damp mugs in the kiln room when the room is already hot, the rapid temperature change will cause rapid drying and shrinkage which can place your mugs at risk of cracking.

Using a fan to move some air in the drying area where you have damp mugs can work if the air movement is not too strong and not aimed directly on the mugs. It is beneficial if mugs are repositioned every so often to make sure that all sides are drying evenly.

 "Damp" is referred to when the clay is almost dry but has slight signs of wetness. At this stage, most of the drying shrinkage has occurred and the clay does not have much remaining movement. Thus, it makes it relatively safe to expedite completion of the last bit of drying. The only exceptions are large forms, in particular flat forms, and mugs with walls that are thicker than 1/2".

Initial Kiln Drying

Complete drying doesn't take place until the piece is in the kiln. This happens when the boiling point of water has been reached (100 degrees C, or 212 degrees F.). This must happen slowly, or the formation of steam within the body of the clay may cause it to burst. For this reason, the early stages of firing are done slowly, and with a kiln vent operating, peephole or lid open for steam to escape. If computerised kiln controller is used, candle the kiln at 180 degree F for several hours - see more details All about firing an electric kiln.

Dehydration

The next change which occurs is at about 350 degrees C (662 degrees F), the point where the chemically combined water of the clay is driven off. This is water that is part of the molecular structure of the clay, not the previously described water that is between the particles of the clay. This drying is completed by about 500 degrees C (932 degrees F). After this point you could no longer mix the dried clay with water to make new wet clay. An irreversible chemical change has taken place, known as dehydration. No shrinkage is observed during this stage.

Burn-off

Another thing which happens up to about 900 degrees C (1652 degrees F) is the burning off of organic and inorganic materials, such as carbon and sulphates. These are the fumes that it is important not to breathe, and the reason a kiln should be well ventilated even during bisque firing.

Quartz Inversion

After dehydration, the next change that happens is Quartz Inversion, which happens at 573 degrees C (1064 degrees F). At this point, quartz crystals rearrange themselves into a slightly different order. A slight and temporary increase in volume occurs at this point. This is why you always need some space around pieces during firing, as they will expand somewhat. Firing should proceed slowly during this Quartz inversion. A large percentage of ware that is cracked during firing happens from fast firing through this stage. The factory set program on electronic kilns usually slow down the firing at this stage for you.

Vitrification

The next stage that happens is vitrification. This is the hardening, tightening and finally the partial glassification of the clay. Vitrification results from fusions or melting of the various components of the clay. The strength of fired clay is increased by the formation of new crystalline growth within the clay body, particularly the growth of mullite crystals. Mullite is an aluminum silicate characterized by a long needlelike crystal. These lace the structure together, giving it cohesion and strength.

Shrinkage happens at the vitrification stage. This is due to diminished size of the particles as they approach fusion and to the closer arrangement of particles in their glassy matrix. The firing shrinkage of a clay is usually about the same as the drying shrinkage. Total shrinkage will usually be about 8-12%.

Clays vitrify at various temperatures depending upon their composition. A red clay high in iron and other impurities might fire to hardness at about 1000 degrees C (1832 degrees F) and melt to liquid at 1250 degrees C (2282 degrees F). A kaolin body which is very free from impurities might not melt until over 1800 degrees C (3272 degrees F)! By mixing the ratios of different types of clays that melt at different temperatures, clay bodies are developed for different firing temperatures.

Melting

If you fired high enough, the clay would first swell up (bloat) then fuse into a liquid which would cool as a glass. Or course in ceramics we don't fire that hot; we stop at the point where we have just enough fusion and hardness for durability, but not too much so we cause melting or deformation of the ware. This point is called the maturing of the clay.

Common Mug Defects

Pinholing and Pitting

Pinholing and pitting are glaze defects in which the glaze comes out of the kiln with one or more pits in its surface. Pinholes are the smallest of these pits.

Reasons for Pinholing and Pitting

All glazes contain volatile materials and will undergo a certain amount of agitation as these burn off during firing. Most pinholes and pits are due to this off-gassing. In a real sense, the glaze became frozen while it was still boiling.

Underfiring a glaze can leave it with pinholing and pitting. It is not surprising, therefore, to find that matte glazes are more subject to pinholing and pitting, since they are glazes made to purposefully be underfired.

Other contributers to pinholing and pitting include high levels of zinc or rutile in the glaze. In addition, if the kiln enters into reduction during the early stages of firing, carbon may be deposited on the ware and can contribute to pinholing and pitting as it later burns off.

Solving Pinholing and Pitting

Solutions include:

• increase the firing time overall
• increase the soak time at the end of the firing cycle
• down-fire (keep the burners on while allowing the kiln to slowly cool) until the glaze has re-solidified
• apply the glaze more thinly
• add a small amount of flux to the glaze
• increase the final temperature you fire to
• reduce the rutile or zinc in the glaze, if any is present
• ensure that, if you are using a fuel-burning kiln, the kiln does not enter reduction during the early stages of the firing cycle.

Crazing

Crazing is the glaze defect in which a network of cracks develop in the fired glaze. These cracks are often very fine, but can also be quite severe. You may first notice crazing of a glaze when removing the pot from the kiln or crazing may develop over time.

Crazing is generally undesirable; however, certain glazes, referred to as crackle glazes, are specifically composed to develop a controlled form of crazing. Crackle glazes should never be used on the interior of any pot that is able to hold liquid or foodstuffs.

Reasons for Crazing

This fault is usually caused by a glaze that is too small for the clay body. The glaze contracts more than the clay body as they cool from their maturation temperature in the kiln. The resulting tension in the glaze causes it to crack. Other common causes are:

• thermal shock due to abrupt and significant temperature changes
• overfiring a mug, which can melt the silica in the clay body, changing its coefficient of expansion
• expansion of the clay body due to absorption of moisture after firing, if the clay body is porous and has unglazed areas
• poor fit between the clay body and the glaze.

Solve Crazing Through Changing the Firing

Crazing may be solved or reduced by firing to a higher temperature or by introducing a longer soak at the end of the firing cycle. However, if you are already firing to a point where the clay body is partially vitrified (for example, porcelain), increasing the temperature or soak may actually increase the problem.

Solve Crazing Through Changing the Glaze

If the crazing is due to a mismatched glaze and clay body, the best solution will be to modify one or the other's coefficient of expansion.

To modify the glaze, try one of the following:

• increase the silica
• reduce the feldspar (or soda or potassium in general)
• increase the boron, or
• increase the alumina.

Solve Crazing Through Changing the Clay Body

Generally speaking, a fairly moderate increase in the amount of silica in a glaze will correct the glaze defect. If, however, there is a marked soda or potash content it may be more practical to modify the clay body, in one of the following ways:

• For low-fire clay bodies, increase the soda or potash
• For high-fire clay bodies, the most practical method of curing crazing may be to increase the silica in the clay.

Mugs For Promotional Purposes

As you can see from the above, there are a whole raft of styles, shapes, print options and finishes available on mugs and this is one of the main reasons why mugs are so popular as promotional giveaways. Their longevity ensures that any branded message or logo will remain around for months if not years to come and a well designed mug which becomes a recipients mug of choice is likely to be used regularly on a daily basis remaining on their desktop providing businesses and organisations with continual advertising. There are few other promotional items which can meet this unique opportunity and offer such a great return on investment.

Additional Resources