Porcelain vs. Ceramic Tile
Is There a difference?

One of the things we are frequently asked about while helping cli­ents select flooring and countertop materials for kitchens and baths is the difference between por­ce­lain and ceramic tile.

People have a sense that ceramic and porcelain tiles are different, and that porcelain tile is somehow the better of the two, but they're not sure how they are different or why porcelain is better.1

So, exactly what is porcelain tile, and is it really better than ceramic tile? Let's see if we can de-mystify porcelain tile a little.

Let's start with this simple, basic fact:

Porcelain and ceramic are not different kinds of tile. They are both ceramic tile.

Ceramic is a word derived from the ancient Greek "keramos" meaning roughly "of fired clay". Porcelain is just one of many varieties of fired clay or ceramic tile. Ceramic tile, including porcelain, is made out of clay, some additives such as feldspar, bentonite, and quartz; and water.

The European Definition of Porcelain

But, not just any old0 backyard clay will do. China clay, suitable for making ceramic tile, contains a high proportion of a mineral called kao­li­nite, and, for that reason is often called kao­lin clay or just kao­lin.

Kao­lin results from the chemical decay of silica minerals, mostly feldspar. Clay that is pure kao­li­nite is bright white, but white deposits are very rare. Most deposits of tile clay are mixes of kao­li­nite and other minerals such as alumina, silica, mica, quartz and iron oxide. Iron, which is plentiful in the earth's crust, is the main source of the orange, brown and terra-cotta colors of most china clays.

The term "porcelain" has been used in Italy at least since at least the late 13th century 2 to describe ceramic tile made of white clay.

The Clay Firing Process

The chemical and mechanical changes that occur during the firing process are enormously complex, and even after 10,000 years for firing clay, not completely understood. A lot of tile making is still art, using trial and error rather than science.


A bisque is fired in four stages. The temperature of the kiln is slowly increased until the desired firing temperature is reached, then the tile is slowly cooled until it reaches room temperature. The higher the target firing temperature and the longer the bisque is held at that temperature the harder and more water resistant the resulting tile.

Stage 1. Drying: (up to 480° F). Residual water removed. Temperature is raised slowly to prevent explosive vaporization.

Stage 2: Dehydroxylation & Oxidation: (480° to 930° F) Chemically bonded or "lattice" water evaporated (dehydroxylation), organic matter burns off (oxidation).

Stage 3: Vitrification: (930° to 2,700° F) Kao­li­nite particles crystallize through several intermediate states as the temperature rises, finally transforming into mullite and cristobalite — elements that give tile its strength and rigidity — while glassy components of the bisque melt and flow into the spaces between the clay particles.

Stage 4: Cooling: (2,700° to room temperature) Tile is allowed to return to room temperature slowly to prevent cracking.

To medieval Italians, or so the story goes, the resulting ceramic products had the the color and luster of the much admired shell of the porcellana cowrie. So, that's what they called it: porcellana.3 The French corrupted the word to "porcelain" and we adopted (the French say "purloined") the French word.

The clay and additives are mixed with water to produce a "paste" which is formed into a "biscuit" or "bisque" — the body of the tile — and heated to a very high temperature (usually between 2,000° and 2,500° Fahrenheit) in a kiln using a process first discovered by early humans about 10,000 years ago.

All clay tile is fired the same way. Folk wisdom has it that porcelain tile is better because it contains a higher proportion of kao­li­nite than red or terra-cotta tile. But, that's nonsense. Pure kao­lin actually produces a relatively fragile and brittle tile.4 It is not the proportion of Kao­li­nite in the bisque that makes a better tile, but the proportion chemically transformed tshrough firing.

Firing drives out water, hardens the clay and fuses the clay particles together through a process known as sintering.5 It also changes the silica in the mix to a crude glass which flows into the gaps between the clay particles, sealing them up. This process is known as vitrification6. If all of the gaps in the tile are filled with glass, the tile becomes essentially waterproof.

How much of the mix is vitrified depends on how long and how hot the bisque is fired. If fired for a long time at a high temperature, more water is driven out and more sintering and vitrification occur=: resulting in a denser, harder tile more resistant to water absorption. Light-colored clays can be hard fired, as can more common red, brown or terra-cotta clays. The color of the clay makes absolutely no difference to the firing process.

So, our first definition of porcelain — the traditional or European definition — is a tile made from white or light-colored clay.

It is the color of the clay that defines porcelain for the traditionalists. The tile may be hard, or not; fired for a long time, or not; and highly vitrified, or not. As used in this traditional sense, the term porcelain tells us nothing about the quality of the tile. It tells us only that the tile is made out of light-colored, porcellana clay rather than a red or brown terra-cotta clay.

The American Definition of Porcelain

The American definition of porcelain is much newer and very different — almost the exact opposite in fact. It results from the particular American experience with fired clay products.

Some Silly Porcelain Myths

A number of widespread myths have grown up around porcelain tile over the years — most of them utter nonsense. Here are a few of our all-time favorite goofy fables about porcelain tile.

Porcelain glaze has a depth and luster not available on ordinary ceramic tile.

The truth is that glaze is deep and lustrous if the manufacture applies a thick coat of lustrous glaze.

It makes no difference what it is applied to. Glaze can be used to decorate and protect clay tile, steel, iron, pottery, china, even concrete — in fact, just about any material that can withstand the firing temperature.

The notion that porcelain glaze is somehow deeper and more lustrous is pure fiction.

Porcelain tile is made from a special "refined" porcelain clay.

Horse apples!

Of course, each manufacturer has its own "secret formula" for tile, but there is no universally recognized special clay mixture used to make porcelain tile. But, this fact has not slowed the many "composition" myths surrounding porcelain tile. These suggest that the clay formula used in porcelain has some special or unique properties.

Here are some of our favorite variations found on the web from folks who really should know better:

“Porcelain tile is … made from a much finer clay than ceramic, composing kao­linitic minerals, quartz and feldspar, covered by spray and shaped by dry pressing the clay dust to form a ceramic material that is then fired at higher temperatures than ceramic.” HouseandHome.com

“The only difference between porcelain tile and regular ceramic tile is that the clay used in porcelain tile is more highly refined and purified. Consequently, porcelain tiles are denser than a standard ceramic tile.” HomeAdditionsPlus.com

“Porcelain tiles are composed of fine porcelain clays and fired at much higher temperatures than ceramic tiles.” FloorFacts.com

Porcelain tiles “ are made up of a sand-like material".” World Floor Covering Association (Evidently blissfully unaware that firing a “sand-like material” produces glass, not ceramic tile.)

It's all total balderdash, there is no special clay formula for porcelain tile.

The installation of porcelain tile requires special tools.

Porcelain requires the same tools as any ceramic tile. No "special tools" are required to install porcelain tile as suggested by some "experts"(See e.g. FastFloors.com). All the tools required are already in any tile setter's tool box.

Highly vitrified tile will take more time to install. It is very hard, saws more slowly and chews up sawblades more quickly. And, because it is less porous, it accepts thinset adhesive less readily. Experienced tilers will use a special extra sticky adhesive on very hard tiles. But, again, ths is true of all highly vitrified tiles, not just those that the manufacturer has chosen to call porcelain. The additional time is not significant and should add very little to the cost of installation.

In the U.S. there are three separate and distinct ceramics industries7: ceramic tile, dishware/pottery, and sanitaryware. There is almost no overlap. Makers of fine china do not make toilets, toilet manufacturers do not make floor tile and floor tile companies do not make dinnerware.

This is a little curious because they all use basically the same raw materials and roughly the same manufacturing process: a clay paste is shaped into a product — a salad bowl, sink, vase, floor tile, etc. — and then fired at high temperature to harden the clay. Then the product is covered with an impenetrable, baked on, glass coating.

In the early days, American tile manufacturers rarely made traditional porcelain tile. If someone actually wanted porcelain tile, it was imported from Europe. But, the other clay industries adopted traditional European porcelain very quickly, and their products soon earned a reputation for being of exceptional quality. By 1917 Lenox had replaced European manufacturers such as Spode and Wedgwood as the purveyor of fine china to the White House. Kohler and American Standard had firmly established the "American standard" for nearly faultless porcelain bathroom fixtures as early as the 1890s.

By the time American tile makers needed a word to describe their better quality tile, "por­ce­lain" was available and already well-established in the minds of the buying public as describing ceramic products of exceptional quality. So, they borrowed it, and porcelain, over time, became the term reserved for better quality American tile.

There is some evidence that American porcelain tile followed the European practice of using only light-colored clay for a few years. But, after a while this limitation began to give way so that by the 1970s the term was applied to any high-quality tile no matter the color of the clay. Today, American porcelain may have a tile body that is red, tan, white, brown — any color so long as the tile is of good quality.

So, for American tile makers the defining characteristic of porcelain tile is the quality of the tile, not its color. The tile may be of any color — color does not matter — but it must be a tile that is considered to be a high quality product.

Full-Bodied & Through-Bodied Tile

The problem with the American definition is that it conflicts with the general public perception of porcelain. While most people don't know that much about porcelain, they do know that it is a white- or light-bodied tile, not red, brown or terra cotta. Consequently, tile sellers often meet with a lot of skepticism when they try to sell red-brown clay tiles as porcelain.

To overcome this problem, American tile manufacturers have come up with a clever distinction between red or brown and light bodied porcelain tile. Light-bodied tile is "full-bodied" or "through-bodied" porcelain. So, the salesman can tell you, with an absolutely straight face, that the wine red terra-cotta tile you are looking at is actually porcelain, it's just not "full-bodied" porcelain — implying that you must be an unwashed country bumpkin for not knowing the difference.

Irreconcilable Differences

So, we now have two almost opposite and conflicting definitions of porcelain:

Rectified Tile

When the guy at the tile store tells you that the tile you are looking at is more expensive because it has been rectified, it sounds vaguely ominous — like someone's been naughty and got sent to the principal's office. Just what does he mean?

Tile that is fired at very high temperature loses most of its moisture, causing the raw tile bisque to shrink. Shrinkage is very controlled these days so that the finished size or "caliber" of the resulting tiles is very uniform — usually less than 1/16th of an inch difference from tile to tile. But, if the tile has to all be exactly the same size, it is "rectified" by cutting all the tiles to exactly the same size on a saw or grinder. This extra step adds a little to the price of the tile

Rectified tile is for special applications and is just not needed for most home uses, so don't pay for it unless your tile installer insists you actually need it.

• The European tradition that refers to any light-colored tile body as porcelain, no matter the quality of the tile; and
• The American definition that refers to any high quality tile as porcelain, irrespective of the color of the tile body.

So, if the tile store clerk claims that the tile you are looking at is "porcelain", what does he mean? Is it a high quality tile, or a light bodied tile? Both? When you buy a tile labeled "porcelain", just what are you buying?

Testing & Rating Ceramic Tile

Although some tile "experts" expound at length on the "special" and "unique" properties of the clay mix used to make porcelain tile, there is, in fact, no such special clay mix (See sidebar). There are absolutely no composition standards at all for general-use clay tile. No one cares about the type or color of the clay used or just how much or how little kao­li­nite, or feldspar, or quartz sand, etc. is included in the mixture. Manufacturers can make tile out of any composition and color of clay, and include any additive that they think will produce a better tile. The standards don't care one way or another.

Nor is anyone particularly concerned about how the tile is made. There are no process standards for general use tile. Any manufacturing method that will work is just fine. The tile can be shaped by machine, or by hand. It can be extruded, pressed or globbed into a form. The tile can be fired using an old tunnel kiln, or the newer roller-kilns, or a kiln at the local high school. With rare exception — for special purpose tiles: the heat-shield tiles on the space shuttle, for example — the standards simply don't give a hoot how a tile is made.

What the standards do care about and do test extensively is how well the tile performs. Standards used to rate fired clay tiles are all performance standards. If a tile performs to a certain standard, it gets rated for that standard no matter what it is made of, how it was made, or what color it is.

Ceramic Tile Performance Standards

Ceramic tile is subjected to a great many tests. It is tested for slipperiness; resistance to cold, heat, and chemical damage; breaking strength, and stain resistance, among others — and we will touch on a few more of these tests later in this article. But, the two most widely used — and certainly the most widely quoted and misquoted — tests of fired clay tile are the wear resistance and water absorption tests.

The wear resistance test, developed by the Porcelain Enamel Institute (PEI) reveals how well the tile resists damage from foot traffic. The absorption test, supervised by the American National Standards Instituted (ANSI), looks at how waterproof a tile is.

These distinctions do not necessarily make one tile better or worse than another, they merely help determine where and how a tile may be used. A tile that absorbs a lot of water should not be used outside where it freezes because the freeze-thaw cycle will crack the tile. But, it may be quite suitable as wall tile around a fireplace where water absorption matters little.

A tile that is not very hard may not work for floors or counter tops, but will be just fine for walls, backsplashes, and hobby crafts where surface wear is not an issue.

PEI Rating: Resistance to Surface Wear

The rating developed by The Porcelain Enamel Institute, is a test of surface wear resistance. It involves applying steel ball bearings and aluminum oxide (the grit used in sandpaper) to a tile. A tile is scored by how quickly it shows visible signs of wear.

PEI Wear Resistance Ratings
Group I
The softest tile. Suitable for walls and hobby crafts only, no floors.
Group II
Residential use in low foot traffic areas. In rooms where there is usually no through traffic, this tile might work. But, in kitchens, where there is often a lot of through traffic, this tile would be suspect.
Group III
All residential, medium commercial, normal foot traffic (interior only). Any bathroom or kitchen, mudroom, laundry room or hallway, but nothing outside.
Group IV
Heavy commercial. Any interior use. Suited for residential floors that get a lot of use and for exterior applications where there is not a hard freeze in winter.
Group V
The most wear-resistant tile. Extra heavy, high traffic, commercial (interior or exterior use).

The classification is colloquially known as the PEI Scale or "Wear Scale". The classifications are numeric. The numbers define the suitable uses for the tile. The higher the number, the more wear-resistant the tile and the more places it can be used.

ANSI Rating: Resistance to Water Penetration

The rating developed by The American National Standards Instituted is a test of resistance to permiability by water. It consists of first boiling the tile for five hours then immersing the tile in cold water for another 24 hours and then measuring its gain in weight from the original dry state. The amount of absorption is stated as a percent of change from the dry weight.8 Four ratings resulted from their studies of clay-fired tiles. These are, from lowest to highest: Non-Vitreous, Semi-Vitreous, Vitreous and Impervious.

A Vitreous tile is less porous than a Semi-Vitreous tile because more of the tiny spaces between clay particles have been filled with glass. A Semi-Vitreous tile is less porous than a Non-Vitreous tile.

ANSI Water Resistance Ratings
Non-Vitreous
Water absorption of more than 7.0% by volume.
Tile for non-wet areas. Around fire­places, for example. Typically intended for walls, hobby and crafts use.
Semi-Vitreous
Water absorption of more than 3.0 percent, but not more than 7.0 percent.
Tile for areas that may get wet on occasion, but are unlikely to see constant or standing water. Back­spla­shes or countertops, for example.
Vitreous
Water absorption of more than 0.5 percent, but not more than 3.0 percent.
Virtually any indoor application including shower walls and floors. Outdoors in areas that do not freeze. (Although some Vitreous tiles will pass the frost test, and can be used outdoors. The frost test is discussed later in this article.)
Imper­vious
Water absorption of 0.5 percent or less.
Any indoor or outdoor application.

An Impervious tile is the least porous — in fact, it is essentially waterproof — but instead of calling it super-vitreous or waterproof, the creators of the test settled on Impervious.

Waterproof would actually be the more accurate term. Impervious (in regular English as opposed to engineer-speak) means more than just waterproof. It also means "impregnable" or "invulnerable". Impervious tile is neither. Hit it with a hammer, it will break. So, when you see or hear the word Iimpervious applied to tile, think "waterproof". That way your mind won't be fooled into thinking the tile is something it's not.

There is a relation between the two ratings, at least for unglazed tile such as quarry or saltillo tiles. Tile that is harder and thus more wear resistant is tile that was fired longer or at a higher temperature, or both. This is also the tile that tends to be more resistant to water penetration.

With glazed tiles, however, the relationship breaks down. If a tile has a surface glaze, the glaze becomes the wear layer, and it is the glaze that is subjected to the PEI wear test, not the body of the tile beneath the glaze. A very hard glaze can be applied to a very soft tile body, so, with glazed tiles it is possible to achieve a high wear rating with a low water-resistance score. More about tile glaze below.

The ANSI Definition of Porcelain Tile

We already have two different definitions of porcelain tile: the traditional or European definition, and the newer American definition. And, just to make things even more fun, there is yet a third generally accepted definition: the ANSI definition of porcelain.

Under pressure from American tile manufacturers and seeking to clarify once and for all time the burning issue of what is a porcelain tile, the American National Standards Instituted in 1988 adopted what it hoped would become the only official U.S. definition of porcelain tile, assigning the traditional color-based definition to oblivion. In its American National Standard Specifications for Ceramic Tile (ANSI A137.1) ANSI declared a tile to be porcelain only if it is:

"…generally made by the dust-pressed method of a composition resulting in a tile that is dense, Impervious, fine-grained, and smooth with sharply formed face."

In 2007 the Tile Council of North America and the Ceramic Tile Distributors Association formed a joint venture called the Porcelain Tile Certification Agency (PTCA) to test and certify ceramic tile as meeting the American National Standard Institute's (ANSI) definition for porcelain tile.

To be considered porcelain under the ANSI definition, the tile must score as “Impervious” on the ANSI water absorption test (see above). If the tile passes this test, it is given a certificate indicating that it is “porcelain” under the ANSI definition of porcelain.

The certification mark may be used on the tile's packaging and marketing materials to show that the tile is considered porcelain as defined by ANSI. So, if you see this mark on the tile box you may be sure that the tile is water-resistant enough to be used in any indoor or outdoor application. Of course, if all you are tiling is a shower wall or a backsplash, this is much more durability than you actually need, so why pay more for it? See “How to Actually Buy Clay Tile”, below.

While this is definitely written in engineer-speak, what it does is define two different tests for identifying a porcelain tile. The first is the same performance test used to rate all tile. A porcelain tile must perform at the highest, Impervious, standard in the ANSI water-absorption test (See above). The second is something rarer — a "process" test. To be called porcelain, tile must be made by the "dust press" method or process.

The dust or "dry" press method is a process for manufacturing tile that has very little to do with tile quality. It is mostly a measure for controlling tile shrinkage.

All fired clay tile shrinks when it is fired and water in the biscuit evaporates. If there is a lot of water, there is a lot of shrinkage. Controlling shrinkage is important if the resulting tiles are to be the same finished size, or "caliber". Caliber is important. If tiles vary too much in face dimensions they are much harder to set in a pleasing, un­iform manner and often require a wide grout line to mask the unevenness of the tile. Tiles that are very similar in finished size can be set with a narrower grout line.

The dust press method does not actually use dust to make a bisque. A little water — about 6% — is required to dampen the clay mix which is then pressed into shape. The pressure helps the clay particles stick together so the bisque retains its shape. With so little water in the mix, shrinkage is minimal and the resulting tiles are likely to be of a more uniform size.

The other, more widely used method, is the extrusion process in which slightly wetter clay is fed into a mold that extrudes a ribbon of biscuit that is then sliced into individual tiles. In the past, shrinkage was a little less controlled using this method.

Encaustic tiles are ceramic tiles in which the pattern or figure on the surface is not a product of the glaze but of different colors of clay inlaid into indentations in the body of the tile. The inlays are typically 1/8 to 1/4 inch thick, and they may be composed of as many as six colors. Because of its thickness, the inlaid pattern remains as the tile is worn down over time, which makes the tiles preferred for public buildings that are heavily trafficked.

Popular in medieval times and used in a number of Gothic cathedrals in Europe until the 16th century, encaustic tiles were revived during the Victorian era and mass-produced by companies in the United States such as the the American Encaustic Tiling Company of Zanesville, Ohio, which was active until 1935.

Minton, Hollins and Company in Stoke-upon-Trent, Staffordshire, England, produced the encaustic tile, pictured at top, installed in the U.S. Capitol building when it was enlarged in 1856. Its successor company, H & R Johnson Tiles Ltd. has revived its production of encaustic tile for the restoration of the Smithsonian Institution as well as the U.S. Capitol building.

Craven Dunnill-Jackfield of Shropshire also revived its production of encaustic tiles in 2000 in its old tile works, now a part of the Jackfield Tile Museum. It now produces encaustic tile for projects throughout the world, including restoration of the English Houses of Parliament.

For a video on the manufacture of encaustic tile using Victorian methods and machinery, see Encaustic Victorian Tile Manufacturing .

However, pro­cess standards are very rare in the tile world, and this one did not last long. Since the standard was first issued, tile chemists and materials engineers have made enormous strides in controlling shrinkage using the extrusion process — primarily through additives, such as calcium lignosulfonate, that make water "wetter"9. Wetter water means less water is needed in the paste. As a result, extrusion now produces tile of very uniform caliber. Recognizing this change ANSI dropped the requirement that porcelain be made by the dust press process in its later versions of A137.1. Any manufacturing process can now be used to make porcelain tile as long as the tile rates Impervious.

The effect is that "porcelain" under the ANSI standard has become just an alternate word for "Impervious", which, as we noted earlier, is just engineer-speak for "waterproof". If a tile meets the criteria for being rated Impervious to water penetration, it may, according to ANSI, be called porcelain.

American tile manufacturers welcomed the new standard with warm applause and great enthusiasm. European, South American and most Asian tile makers greeted it with a great big yawn, and cheerfully ignored it.

Imported tile is generally rated under a different set of standards published by the International Standards Organization (ISO), which had until recently, very wisely, stayed out of the fray over what is or is not porcelain tile. Consequently, imported tiles were free to adhere to the European definition that declared tile to be porcelain if made with light-colored clay.

The ISO Definition of Porcelain Tile

But, not having an "official" definition of porcelain caused problems for the ISO. The U.S. market for ceramic tile is huge, about equal to the entire continent of Europe, so most foreign tile manufacturers want to export to the U.S. But, in order to get an "official" porcelain designation for their qualifying tile, they needed to be certified by ANSI. This started costing ISO customers as more manufactures shifted to ANSI for tile certification.

In 2012 the ISO caved in. The standard that defines and classifies ceramic tile (ISO 13006) was revised to include a definition of por­ce­lain tile — adopting with minor variations the same definition as ANSI — a tile may be called porcelain if it absorbs less than .05% water by volume, or is, in other words, rated Impervious in the ANSI water absorption test. (Actually, the ISO has its own slightly different, somewhat less stringent test — why let the ANSI guys have all the glory, right?)

So, it was a long road but everyone finally agrees that the old color-based definition of porcelain is extinct, and the term porcelain will be reserved only for high quality tile in the American tradition that meets either the ANSI or ISO standard for Impervious tile. Right?

Not a chance! Sorry! Wish it were that easy.

On some tiles a thin additional layer or “slip” of clay mix called “engobe” is applied to the tile body. Engobe acts like a primer for the finish glaze coat. It is a usually light fine-grained clay which may be colored with various metal oxides to more or less match the final glaze color. This helps intensify the color of the glaze and keeps a dark tile body from showing through a lighter glaze. It is also used to fill in any minor imperfections in the tile body so the finished glaze is smoother. Very glossy tiles almost always include an engobe layer.

After the glaze is fired, a tile can be given a little polish to improve its shine. Some tiles are highly polished, a delicate operation since it is easy to cut completely through the glaze and expose the softer engobe layer which can scratch and stain easily. High gloss tile should always be tested for stain and scratch resistance before buying.

If ceramic tile was required to be graded by law, then a standard defining porcelain would eliminate any issue about what is or is not porcelain tile. The question would then be which of the competing standards to use. But, both ANSI and ISO standards are completely voluntary. No tile maker is bound to observe them. And, many don't. Ceramic tile may legitimately be called porcelain even if it is not "Impervious" as long as it does not claim to meet the ANSI (or ISO) standard for porcelain.

So, where does that leave us? Actually, just about where we started. The attempt by the standards organizations to create a universal standard for porcelain has really done very little effect except to make a confused situation even more confusing.

Many tile makers choose to ignore these new standards, especially traditionalist manufacturers that still adhere to the older color-based, definition of por­celain tile. And, while American tile manufacturers have tried mightily to get the ANSI definition enacted into federal law, our Right Honorable Congress-Persons have so far been busy with much weightier matters — primarily getting re-elected and calling each other names10.

As the situation now stands, the use of the word porcelain to describe ceramic tile has been left almost entirely to the discretion of individual tile manufacturers. Whe­ther they have their tiles tested and certified under either ANSI or the ISO standard as porcelain, and how and when they use the term to describe their tile is completely up to them. As a result, when the word appears on a box of tile it is often not possible to discover what it actually means. It may legitimately mean ...

A high quality tile of any color,

OR

A light bodied tile of any quality,

OR

A tile rated "Impervious" under ANSI standard A137.1

OR

A tile meeting the definition of porcelain under ISO 13006.

So does the word "porcelain" printed on the box actually help you select a suitable tile? It tells you nothing about the color of the tile — porcelain in the American tradition can be of any color. It does not reveal anything about the quality of the tile — porcelain in the European tradition may be of any quality. In fact, it does not tell you much of anything useful about the tile. The term "porcelain" is so ambiguous as to be useless. If the tile meets any of the many definitions of porcelain, and its manufacturer elects to call it porcelain, then it is — legitimately — porcelain.

Our best advice: ignore the term porcelain altogether when shopping for clay-fired tiles. Disregarding the dubious distinction between porcelain and other ceramic tile actually makes choosing tile much, much easier and a lot less confusing. To find out how to actually buy the right ceramic tile for your project, continue on to the next section, How to Choose the Right Ceramic Tile.

Footnotes
1. This confusion is not new. Over 100 years ago William Burton was already complaining that the word porcelain was being applied to such diverse materials that it had lost all meaning. See: Burton, William, Porcelain, Its Nature, Art and Manufacture, B. T. Batsford Ltd., London 1906, pp. 47–48. Available on line and as an e-book, courtesy Google Books.
2. The Venetian explorer, Marco Polo is reputed to have been the first to use the word porcellana in print to describe ceramic pottery c. 1298.
3. No one has yet explained why the shell of the porcellana cowrie was so admired. It looks more or less like every other sea shell.
4. Kaolin needs to be combined with other clays, such as ball clay.to improve elasticity and strength. Ball clays typically contain mica and quartz as well as some kao­lin. They are fine-grained and plastic in nature which makes them ideal for modifying the very brittle pure kao­li­nite. Ball clay deposits are relatively rare, mined primarily in the Eastern United States and in Devon and Dorset in Southwest England.
5. Sintering is normally defined as a process of making a powdered material coalesce into a solid mass by heating it but not liquifying it. The clay particles in the bisque do not melt, but do get hot enough to stick together. In metallurgy it is a cheap and fast way of forming metal without having to cast it in liquid form. The resulting product is not nearly as strong, but so-called "clinker casting" is suitable for things that do not have to be particularly strong such as cabinet knobs and light-duty hinges.
6. The process of transforming a clay paste into ceramic tile through firing is one of the most complex in industrial chemistry, involving a number of what chemists call "structural transformations" of kao­li­nite and silica materials. Kao­lin becomes metakao­lin at about 1,100° F begins to form mullite at about 2,000°. As the temperature continues to rise, silica in the paste is is transformed into molten glass. Mullite and glass in its various forms are the major constituents of fired ceramics. The Kao­lin clay transformed into mullite gives the tile its characteristic strength, structure and shape while the glass helps it resist water penetration.
7. We have excluded brick making from the roundup since bricks are seldom glazed.
8. The formula is (Ws - Wd )/Wd x 100 where Ws is the saturated weight and Wd the dry weight of the tile. For example: A tile that weighs 4 oz. dry and 4.25 oz. saturated has an absorption rate of ( 4.25 - 4 )/4 x 100 or 6.25% and would be classified as Semi-Vitreous tile.
9. Everyone knows that water is composed of one oxygen and two hydrogen atoms tightly bound together. But, hydrogen atoms in the molecule also bind weakly to oxygen atoms in neighboring molecules. This weak bond tends to keep water molecules together, which is why water tends to form drops. Any additive that weakens this oxygen-hydrogen bond is called a surfactant and has the effect of making water "wetter". Common household surfactants are soap, dish washing liquid, shampoo and toothpaste.
Wetter water reacts with its environment more readily than untreated water, which is why soapy water cleans better than plain water. Wetter water also cools automotive engines better, is more quickly and thoroughly absorbed into soil (reducing the amount needed for irrigation), puts out fires faster (the foam in many fire extinguishers is just wetter water), and is thought by some to be healthier than regular water because it "improves hydration at the cellular level" (a notion that has never been proved, by the way).
10. "In my many years I have come to the conclusion that one useless man is a shame, two a law firm, and three or more a Congress." -- Attributed to John Adams, second President of the United States, circa 1802. Obviously, not much has changed since the country was founded.

Rev. 01/22/18