Through the ages clay has been the material used to shape into tiles, used for both decorative and functional purposes in the home and work place. Ceramic tiles have both a practical and romantic history, spanning the ages from the temples of Ancient China and Japan, the palaces of the Tigres-Euphrates Valley, through the homes and mansions of Europe to the Americas. In Italy, the production of ceramicware came to full bloom in the Middle Ages with strong Roman and Arabic influences, and so universal was the use of tiles that an old Spanish proverb says that only an extremely poor man lived in a ‘house without tiles ’.
Modern ceramic tiles are essentially the same as the ancient forms, but todays technology has enabled ceramic tiles to be highly decorative as well as functional whilst still protecting, hygienically and physically, the surfaces they cover. Ceramic tiles for floor and wall covering, once only within the reach of the affluent, are now affordable and indispensable to everyone. Affordable because of their relatively low cost when compared to that of other building materials; indispensable because their hygienic and physically resistant properties make them an obvious choice for areas of domestic, commercial and industrial situations where these features are desirable.
Ever-changing fashions and requirements place increasing demands on the manufacturers to improve their technology and, as a result, consumers are now being increasingly assailed with technical terminology that threatens confusion in the market place. This article, therefore, is an attempt to clarify and explain what is being offered and to be a vital and definitive guide for prospective buyers.
The body of the tile
The main component used in the body of the tile (otherwise known as the Bisque) is clay. For the purpose of this article we will refer only to the body as ‘the clay’ and not concern ourselves with other components usually mixed with the clay such as fluxes, silica, and other raw materials which are of no real concern to the layman. Tiles are thin, flat slabs of clay, which are shaped and then dried and fired at high temperatures. Tiles are shaped by pressing, extruding, casting or even by hand. There are numerous types of clay utilised; in particular, the red bodied tiles are trademarks of Italian manufacture. What is most important to the would-be user or specifier, however, is to understand the requirements for wall as opposed to floor tiles. We will now concern ourselves with the following main manufacturing processes:
Bi’ meaning twice and ‘Cottura ’ the Italian word for cooked. We therefore understand that this method refers to tiles that are twice cooked or fired. The clay is pressed into the required size and shape and fired in the kiln at a temperature of only about 700-800 degrees Celsius. This relatively low temperature (in tile firing terms) ensures that the shape remains uniform with little or no warping, with near to perfect calibration, characteristics most important when considering the aesthetics required for wall tiles. Once the tile has been baked it is coated with the glaze, then re-baked. The disadvantages of this method of manufacture are that the clay is fairly porous and the body strength is not maximised. It is recommended, therefore, that this type of tile is used indoors, particularly for walls. For the layman it is easy to recognise this type of tile as follows: • the colour is normally a very light brown/pinkish colour. • the back of the tile is characterised by protruding nodules. • water poured onto the back of the tile will be readily absorbed. It is also pertinent to point out that some clays are Kaolin based. This body has the same characteristics of the above but is in fact white in colour and normally used for smaller dimensioned tiles such as 15cm x 15cm, 15cm x 20cm and 20cm x 20cm.
Here we have a tile that, as its name implies, is ‘once cooked.’ This method was originally conceived to enable the tile to be fired once at a very high temperature (approx.1200 degrees Celsius). This high firing causes movement in the clay while in the kiln and one result can be that the tiles warp and can become unevenly shaped. Shrinkage also becomes more noticeable and sizing irregular. Why, therefore, fire at such high temperatures? The answer is simple: Porosity and strength
Monocottura tiles have a much greater mechanical strength than Bicottura tiles and are thus ideal for use in heavy traffic areas and where the possibility of impact is greater. Also, these tiles are less porous and can be used externally, particularly where frost conditions prevail. It is important to give a word of warning, however, in this respect: a tile, although classified as monocottura, might not be fired at a very high temperature and therefore could still be porous. One must be very careful to ascertain from the seller or manufacturer whether the tile offered is infact suitable for frost susceptible areas.
Advantages of monocottura tiles:
Advantages of monocottura tiles: single high firing and near-vitrification makes the monocottura tile frost proof. Good mechanical strength enables this tile to resist heavy impact, such as might be encountered in industrial situations as the glaze is applied directly to the raw, unbaked clay, it penetrates deeper into the clay and the resulting fusion is superior to that of the bicottura tile, the glaze is then less prone to chipping,etc. This tile is usually produced for flooring purposes and glazes used are normally harder than those used for wall tiles. Early monocottura manufacturers employed the use of red clays, from which good results were more difficult to obtain. Later technology perfected the white body monocottura, which has resulted in a more perfect tile. Therefore, although perhaps more expensive, the purchase of white bodied monocottura tiles will normally be more advantageous.
iii)Porcellanato or through-bodied tiles
Porcellanato tiles, also sometimes known as through bodied tiles, came to the fore in the market place in the 1980 ’s and were directed particularly at commercial and industrial applications. This tile is of the monocottura family by nature, as it is only once fired. However, this tile has no glazing and derives its name as a through-body tile because the surface material runs through the entire body - the abrasive factors are thus very low. This means that, because the tile will not show wearing, it is ideal for high traffic areas such as shopping malls, factories and banking halls. Although the first models of this type of tile were very ordinary and uninteresting, modern technology has given rise to a vast array of colours and finishes such as the stone look, the granite look and the marble look, amongst others.
The glaze on the tile has many important features, which include decoration, colour, non-porosity as well as cleanability, thus ensuring a hygienically approved product. Besides the above important features, it is also worth remembering that different glazes have different strengths.
This refers to resistance to abrasion and scratching. To understand these characteristics is to appreciate exactly what application the tile is needed for. Domestic or industrial? Wall or Floor? For wall tiles we have to appreciate that aesthetics such as colour and design are of sole importance, but for floors we have to be more circumspect. The intended application must be carefully considered and an adequately glazed tile employed. The wearing qualities of tile glazes is normally measured through the method devised by the Porcelain Enamel Institute of the USA for classifying ceramic tiles, according to their resistance to abrasion. The test is commonly known by the initials PEI and classifies tiles into four distinct groupings:
Floors subject to high traffic but protected from abrasive and scratching agents such as sand, gravel, etc. In general, these tiles may be used in bedrooms, bathrooms and private dwellings.
Floors exposed to medium-light traffic but protected from abrasive and scratching agents such as sand, gravel,etc. In general these tiles may be used anywhere in private houses, except kitchens.
Floors exposed to medium-heavy traffic but protected from abrasive and scratching agents such as sand, gravel etc. In general, these tiles may be used in all kinds of private rooms, including kitchens, patios, hotel-rooms and related facilities, hospital-rooms, etc
Floors exposed to medium-heavy traffic and not protected from abrasive or scratching agents such as sand, gravel,etc., therefore also rooms to which there is direct access from outside. In general, these tiles may be used in restaurants, hotels, shops, schools, offices, hospitals, etc., with the only exception being the area beneath desks and the cash counters of public places.
The resistance to surface scratches is normally measured by means of a hardness test. This test applies to the theory that only a harder substance can scratch a softer one. For example, only a diamond can scratch a diamond. The hardness scale is qualified therefore as follows:
The ratings are from 1 to 10 and it is easily seen, therefore, that a tile with a rating of 4 will be relatively soft and will scratch more easily than say a tile with a rating of 8.
It is the objective of all manufacturers to produce as big a percentage as possible of perfect or near-perfect ceramic tiles. This, naturally, is not always achieved and most firings will produce their quota of second and third grade tiles and some manufacturers even include a fourth or ‘stock ’ grade of tile in their lines. It is desirable that prospective buyers and users of ceramic tiles should be aware of exactly what they are buying and the following points offer helpful guidelines. These guidelines are divided into two sections, bicottura and monocottura.
This is a tile with a near-perfect glazed finish. Grading allows some minute specks in the glazing but these should not be big enough to be noticeable.
Small blemishes are allowed, but these should not be much larger or more noticeable than a pinhead. Pinholes may also be present and in the case of printed patterns on the tile, these might be slightly smudged or irregular. No warping, size difference or chipping should be in evidence and generally speaking, a second grade tile is normally quite acceptable if bought from a reputable supplier. Some shade difference can also occur.
In this grade of tile rather large blemishes occur and chipped products are not uncommon. The more seriously defective tiles can be used for cutting.
These tiles have more noticeable defects, such as large spots, marks, pinholes and chipping. Unless a special use for this grade of tile can be found, it is best left alone.
Most of the faults and defects found in bicottura ceramic tiles are also found in monocottura. However, the following flaws may also be found in monocottura tiles, which are of the highly fired variety.
Tiles of this premier grade bought from a reliable manufacturer are usually calibrated and variations in size are normally minimal. Sizes should vary by no more than 0,75mm either way. There may be an insignificant amount of warpage but this should never be noticeable and should not affect the laying or appearance of the laid tile.
This grade is not normally calibrated and larger size variances can occur as well as a higher degree of bowing or warpage.
Difference in size and the warpage factor can be very noticeable but with correct selection and laying, certain areas such as courtyards and other outside areas where perfection is not the criterion, could well be tiled with products in this grade.
International markings for grading tiles
The following explains normally accepted international markings and how to read them to determine the tile grades as marked on their boxes.
Red is the keynote here. Either the box is red or the printing on it is red. Another way of signifying first grade is with crossed arrows, usually stamped in red. Some factories also use a figure 1 inside a circle.
Blue is the colour to watch for here. Blue boxes or boxes printed in blue. Other indications of this grade can be a large dot in blue or black or a circle around the figure 2.
Tiles of this grade are usually in green boxes or have green printing on the box. If the sign method is used it will depict a triangle in blue, black or green.
These tiles are usually in plain white or brown boxes with no printing or other markings on them, other than their code number.
Useful hints about tiles
When buying highly fired tiles, always check the calibration in the case of first grade products to ensure that all the boxes show the same calibration figure. If uncertain, do not hesitate to ask the supplier to check them for you. Always make sure that you are buying the correct strength of tile for the area you intend to use it in. This is particularly vital in the choice of tiles for flooring.
When laying monocottura floor tiles, always lay with an open joint. The size of the joint depends on the variation in calibration. It is always best to lay out a metre or two on the floor before fixing in order to establish the correct gap to leave for that particular tile. The larger the size variation the larger the gap should be. Remember that the gaps have to be grouted and that this will tend to hide any variation in size. Always make sure you are getting the grade of tile for which you are paying. Ask what grade the tile is and then check the boxes to make sure they tally. Before accepting delivery, check the shade of the tile on display to ensure that the tile you have chosen is the same, or similar, to the one you receive. This is important as batches of the same tile from different firings can have different tonalities of shade.
Before taking delivery, and particularly before laying the tiles, make sure that all boxes contain the same shade of tile when buying the first grade boxes. Each box is marked with the shade number and it is an easy matter to check the boxes to see that the shade numbers are the same. The shade number is usually shown next to the word ‘tonality ’. If you cannot find the number referring to the shade then always ask your dealer to check it for you. In conclusion it can be said that there is a ceramic tile for every application but it is of paramount importance that the user is satisfied that the technical characteristics of the intended tile fully meet the specifications required for the application the user has in mind. Only by dealing with professional people can the user be assured of the correct information. Reputed manufacturers can normally be called upon to produce written technical information pertaining to a specific tile.