Porcelain vs. Ceramic Tile, Part 2 How to Choose The Right Ceramic Tile
One of the things we are frequently asked about while helping clients select flooring and countertop materials for kitchens and baths is how to choose the right ceramic tile.
There is a lot of confusion over the differences among the grades of tile suitable for various applications.
Some of the confusion is intentional. Unscrupulous tile sellers benefit from the myths surrounding porcelain tile to load unsuspecting consumers with more expensive porcelain tile when the characteristics of porcelain are just not needed.
In Part I of this article, we explained why the label "porcelain" on a box of tile does not help you select the right tile for the application you may have in mind.
The term porcelain is used in so many ways to mean so many things that it has lost its usefulness and merely causes a great deal of confusion.
Glaze vs. Enamel
In the tile world, the glass finish applied to ceramic tile is called the "glaze". In the bathroom fixture industry, the glass finish is called "enamel" — hence the term "enamelware". The two terms mean exactly the same thing, but there is a historical basis for the different names.
The traditional term, going back to the 15th century in the jewelry-making trades, for glass applied to metal was "enamel". The process involves grinding glass into a powder (called "frit") which is applied to metal that has been fired hot enough to melt the frit. It is not only beautiful but is the ideal material for protecting metals from damage by water.
With something small like a brooch or finger ring, enameling metal is fairly simple, but what about something the size of a bathtub? Industrial-scale enameling was not possible until the 1880s when American Standard and Kohler Co. separately developed processes for coating large iron objects — giving birth to the modern bathroom fixture industry.
Following the old tradition, the glass coating was called enamel. When sanitaryware manufacturers began applying over-glaze to ceramic bath fixtures like toilets and sinks, rather than iron fixtures, like tubs, they kept the term "enamel" instead of adopting the pottery term, glaze.
Enamelware was durable, very sanitary, and easy to maintain; and became an instant hit with American homeowners of the late 19th-century who were avid for anything hygienic and sanitary. (See Arts & Crafts Styles: Craftsman, Prairie and Four-Square Architecture for more information). They were so superior to the earlier wood and metal products, that the old products were literally driven from the market within a decade.
Paint manufacturers soon took note of the reputation of sanitary enamelware in the public mind, and seeking to capitalize on the name, started calling some varieties of their more durable paints "enamel".
Fixture manufacturers were a little miffed when the paint guys tried to steal their glory and fought if for years. They lost. Paint companies can call their paint "enamel". Today, sanitary fixture manufacturers refer to their glazing as "porcelain enamel" or "vitreous enamel" to distinguish it from that other stuff the paint companies sell.
The confusion is not new. The English ceramicist, William Burton, a chemist for Wedgwood Pottery, writing in 1906, noted that the term had already been applied to such diverse materials that it had "lost all meaning". 
We suggest that you ignore the term porcelain when shopping for fired clay tiles. It does not provide you with any useful information.
Pay attention solely to the tile's ratings. These ratings tell you everything you need to know about a tile to judge whether it is suitable for the use you have in mind, and help you avoid paying for features you simply don't need.
Glazed vs. Unglazed Tile
Most fired clay tile is glazed. Some isn't. Quarry and saltillo tiles, for example, are not glazed. The color of the tile body is the color you see, so the color of the clay makes a difference.
But, most ceramic tiles have a coat of opaque, glassy material on the face of the tile that we call the glaze.
There are emerging one-step (or "monocottura") processes in which the tile and glaze are fired at the same time. Traditionally, however, glaze is applied after the biscuit is fired, then the tile is fired again to set the glaze. This is called a "bicottura" or double firing method. And, in fact a tile may be fired several times to set different colored glazes or elaborate patterns. One color is fired, then the next color, and so on, until the pattern is complete.
It's this glaze that gives the tile its color and texture, not the composition of the tile body. The glaze is the tile's paint.
Think of wood siding. You don't see the wood in your siding. What you see is the coating on the wood — the paint. Most likely you don't even know what kind of wood was used for your siding, or if it was wood at all. It may be Masonite® or one of the more contemporary products, fiber cement siding, for example, which is a wood fiber/ portland cement composite material.
The same is true of glazed tile. Once tile is installed you can't see the tile body, you see only the glaze. So what difference does it make if the tile body is white, sand, terra-cotta, or puce?
If you need to go around a corner where the edge of the tile will be exposed, there are special edge tiles made just for that purpose. And, if you can't find a matching edge tile, your tile setter has a few tricks to disguise the edge.
Glazing was, until just a few decades ago, more art than science.
It took many years of study under a master glazer to learn the combinations of minerals and additives, the heat variations, and the amount of time in the kiln that would produce the many colors, lusters, and textures on tile and pottery.
Today glazing is mostly science.
Consistent glazing compounds and standardized processes make it easier to reproduce the same finish over and over with great accuracy.
So, while it is still desirable to get all of your tile from the same firing to guarantee color consistency, it is not nearly as important as it once was. Glazers can match most basic glazes from batch to batch so that any differences are invisible to the naked eye.
By the way, we have heard mentioned from time to time that the glaze on good porcelain tiles has more "depth" than that on ceramic tiles, but that's just another nonsense myth.
Glaze is "deep" if the glazer applies a thick coat of glaze, it makes no difference what kind of backing material the glaze is applied to. Even concrete blocks can have a "deep" glaze. This is yet another of those fables surrounding porcelain tile that just won't go away.
The Ratings Tell a Tile's Whole Story
Whether your choice is a glazed or an unglazed tile, there are only two things you actually need to know about any fired clay tile.
The first is the most important. Do you like it? Does the size, pattern, price, and color all work for you?
If so, it's time to check the tile's ratings.
All the other information you need to assess the usefulness of the tile is printed somewhere on the box or a spec sheet, often in the form of icons. The ratings tell the tile's whole story including where and how it may be used and if it is suitable for the application you have in mind. If a tile is rated for the application you intend, it will work irrespective of its color or whether its manufacturer chooses to call it porcelain or not.
Let's see just what these ratings can tell us.
Tile grade is the result of a visual inspection.
The range is 1 to 3, the lower the number the better the tile.
Almost all the tile in a tile store will be grade 1. Sometimes you will find grade 2 tile on a "special purchase" sale — often at quite the discount.
Grade 1 (Standard Grade) tile exhibits no obvious imperfections when visually inspected at a distance of 3 feet.
Grade 2 (Secondary Grade) tile shows no visible imperfections at a distance of 10 feet.
Grade 3 (Cull Grade) Tile with major aesthetic problems including wide variations in tone and sizing.
Grade 2 is just fine for many applications. The durability of a grade 2 tile is usually not suspect, it merely has visible imperfections. We sometimes use it in historical renovations to simulate 19th-century tile that often had many visible flaws.
Grade 3 (Cull Grade) tile is rarely seen in retail stores. In fact, most tile companies just break up and discard culled tiles. They can be used in some applications, but not by DIYers. Let the tile professionals buy this tile. They know where and how to use it, or not use it as the case may be.
This is the result of the PEI wear test that we introduced in Part 1. Many manufacturers order this test only on floor tiles.
The higher the rating, the more wear-resistant the tile. A tile used as flooring or on a countertop should be rated at least in Group II (light traffic floors). A higher rating is needed for floors with medium or heavy traffic.
Floor tile should also be at least 1/4" thick. Thicker is generally better.
If the tile is glazed, then it is the glaze coating that is tested. If the tile is unglazed, such as in quarry tile, the tile body itself is tested. The tests are slightly different
If this rating is missing, the tile is probably not intended for floors — and will usually say so right on the box.
This is the score the tile received on the ANSI test for resistance to water penetration (See Part 1 for more information.). A tile that is installed outdoors where there is a real Winter should not absorb water. Otherwise, water trapped within the tile may freeze, fracturing and cracking the tile.
Almost all tiles rated impervious will work outdoors, but so will some vitreous tiles. Look for the frost rating (see below).
For indoor applications, semi-vitreous and vitreous are strong enough for floors and non-vitreous for walls. Pmpervious is too much tile, and a waste of money spent on features like freeze-resistance that you don't need inside the house.
All tiles called "porcelain" do not necessarily rate "impervious". Don't rely on the word "porcelain" on the box, look specifically for a frost rating or a W.A. score of "impervious". Some hardy tiles intended for use outdoors will have both.
If there is a PCTA certification of "porcelain", then the tile has been tested for water absorption and found to be impervious, and this certification is a substitute for a W.A. rating of "impervious".
For floor tile, this Coefficient of Friction (COF) ranking is important. The test establishes how much force is required to move an object across the face of the tile, dry or wet. It tells you how slippery a tile is. The higher the score, the more slip-resistant the tile.
Tile COF can be rated "wet" or "dry". For a general floor, both
require a dry rating of 0.6 and above for a dry floor to be considered "safe".
For a bath or kitchen, where the floor is likely to get wet, a wet rating of 0.6 or greater is required and 0.7 or higher is better. Some tiles specifically designed for wet floors are rated above 0.85 wet — a floor you could not slip on even with great effort.
COF is an important consideration, especially in wet areas. One of my neighbors ignored COF when selecting impervious tile for his front stoop, and now, on wet days, you have to tip-toe over the tile with a death-grip on the handrail to avoid great bodily harm.
There are some problems with this test, however. The approved testing process (ASTM C1028-07), involves pulling a weighted board with a Neolite® (rubber) sole (used to simulate the bottom of a shoe) along the surface of a test tile.
This is called a static slip test. However, the way we walk involves more than just slip-sliding along. There is both downward and outward force applied with each step we take. The static slip test tells us nothing about the effects of these forces.
To test downward and outward force, what's needed is a kinetic or "dynamic" test. There are several dynamic resistance tests. The most widely accepted test in Europe is the ramp test (DIN 51130) developed in Germany. The test involves a person walking along a ramp of tiles that are being tested. The ramp's incline is increased to a point where the person slips
Obviously, since people react differently when anticipating a slip — including changing their stride and walking more carefully — and some acrobatic individuals can airily skip down slippery slopes that would kill us clutzier types, this test has some basic reliability problems. It has not been widely adopted outside of the European Community.
The static slip test has its own reliability problems.
It often misreads the slipperiness of wet surfaces, showing them safe when in fact they are not. Both the Ceramic Tile Institute of America, and the Tile Council of North America have acknowledged that the test is inadequate for assessing the slip safety of ceramic tile.
The most promising replacement is the Dynamic Coefficient of Friction test (DCOF/ACU). The testing protocol requires a BOT-2000 drag-sled meter that crawls along the tile under its own power at a constant speed measuring the resistance to slip of a standardized piece of rubber loaded into the bottom of the machine. The minimum acceptable DCOF test value for floor tile using this test is 0.42 wet or dry. Below .30 is considered unacceptable for floor tile.
At the moment many tile companies are using and reporting both the static slip test and the BOT-2000 test, but the expectation is that eventually the static test will be discontinued altogether.
Breaking strength is important for floor tiles. When you step on a tile, you don't want it to snap. The tile industry uses ASTM C648-04 to determine the strength of the tile. A force is applied to an unsupported portion of the tile specimen until it breaks. The tile's breaking strength is stated in pounds of force applied.
A tile for use on floors must have a breaking strength of at least 250 lbs. Higher is better. And, if Uncle Georgie, even after his last diet, still weighs in at a crushing 380 lbs., higher is definitely required.
This rating is typically not printed on the tile box or datasheet. It is simply assumed if the tile is intended for floors. If you want the actual rating, you may have to telephone the tile company to get it.
Chemical resistance is measured using ASTM C650-04. A tile sample is placed in continuous contact with a variety of chemicals for 24 hours. The surface is then rinsed and examined for deterioration and visible color or texture chandes.
This is a pass/fail test. If there is any change in the tile, it fails and is usually not even put on the market or it is sold subject to limitations printed on the box or specifications sheet.
|Stain Grading Table|
|Class||Stain completely removed with ...|
|2||Solvent specific to the staining agent|
|1||Stain not removable|
The "Frost" icon tells you that the tile can withstand repeated cycles of freezing and thawing. And, that's how it's tested. It is put through multiple freeze/thaw cycles to see if it will show damage.
The result is reported as pass/fail. If it cracks or spalls, it fails.
The body of a tile rated "impervious" in the water absorption test will almost always withstand repeated freeze/thaw cycles, but this may not be true of the glaze. Some glaze will "craze", i.e. form small cracks. This frost test subjects the glaze as well as the body of the tile to freeze/thaw cycles to see if both parts of the tile can withstand the stresses of Winter.
Some tiles rated "vitreous" in the water absorption test will pass the frost test, but don't rely on the vitreous rating alone. Look specifically for the frost test.
If there is no frost icon, it means that the tile is not suitable for use outdoors in any place where it might freeze.
Tone & Shade Uniformity
|Tone & Shade Table|
|V1 (Low) indicates low visible shade and texture variation within each carton.|
|V2 (Medium) indicates a medium shade and texture variation within each carton.|
V3 (High) tiles show distinguishable differences in texture and pattern within each color.
V4 (Random) file contains random variations of shade and texture within each carton.
The tone and shade rating is an indication of how much variation there is in the color and shade of the tile.
The range of ratings on this V-scale is V0 to V4. V0 (monochromatic) indicates no visible color or shade variation among the individual tiles.
Grades V3 and V4 need to be continually blended during installation to insure optimum appearance. This generally results in higher installation costs.
For grade V3 or V4, examine more than one tile, and more than one box of tiles to see if you like all the various tones. There may be considerable variation among cartons.
Some Informal Tile Quality Tests
How did we tell good tile from not-so-good before there were these helpful package ratings? There are traditional tests that can be used to help judge the quality of a tile.
Here are some rule-of-thumb tests that have been around for many years.
The Weight Test
Hard fired tile is generally denser and therefore heavier than softer tile. You probably can't judge comparing single tiles, but heft the boxes. The heavier box is usually the harder tile.
The Ring Test
Hold a tile loosely between your thumb and forefinger at one corner and let it dangle. Snap at an edge near the middle of the tile with your fingernail.
A well-vitrified file with a high crystallization content will ring like a lead-crystal glass. The sharper and higher the ring, the more vitrified the tile. If it goes "thunk", think of it as wall or backsplash tile.
The Color Test
Hard-fired terra-cotta tile is generally browner. We don't know why. It may be the composition of the clay or the fact that high firing turns the clay browner. And, it's not always true. But, generally, hard-fired tile is browner.
(Completely Unofficial and Entirely Unapproved)
Ad Hoc Kinetic Slip & Fall Field Test
The COF rating indirectly tells you whether a tile is slippery, but it's not 100% reliable. As we indicated above, the most common tests of slipperiness all have some reliability problems.
The most accurate test is our Ad Hoc Kinetic Slip & Fall Field Test. It is a simple two-part test.
1. Carefully place several tiles on a hard, level floor, well supported so they do not tilt, rock or wobble.2. Walk on the tile several times, back and forth.
If you slip and fall, the tile fails the test. Don't buy it, and call your lawyer.
If you need a wet kinetic slip test, toss some water on it first. The tile store clerk will probably go a little batty when you do this, but if you want to be completely satisfied that a tile is not slippery, do it anyway.
The Informal Mohs Scratch Test
In 1882 mineralogist, Friedrich Mohs developed a table of relative hardness of minerals which has since become well-known as the Mohs Scale. Each mineral was given a number, with talc, the softest mineral ranked as 1, and diamond, the hardest, ranked 10.
Mohs Table of Relative Hardness
|Rank||Mineral||Test With …|
|6||Orthoclase||Case hardened Steel File|
|4||Fluorite||Pocket Knife/Steel Screw|
Each mineral in the table will scratch any mineral ranked lower in the table, but will not scratch any mineral ranked higher.
Since many of these minerals are a little hard to come by in daily life — just what the devil is fluorite or orthoclase anyway? — substitutes are generally used for the informal Mohs scratch test. These are shown in the table.
The substitute will scratch any mineral lower than its place in the table, but will not scratch any material higher than its place in the table. So, a copper penny will scratch gypsum, but not fluorite, whatever that is.
Wall tile should be at least 4 on the Mohs Scale. You should not be able to scratch it with a copper penny. Floor tile should be at least 6 (7 is better in high traffic areas). You should not be able to scratch floor tile with a piece of ordinary window glass.
If even a case-hardened steel file does not scratch the tile, it's some kind of alien super-tile, possibly escaped from a secret government lab in Area 51. Test for radiation!
Relative Hardness of Flooring Materials
|8-10||(No flooring is this hard)|
|6-7||Glazed Ceramic Tile|
Engineered Quartz Countertop
|Case-Hardened Steel File|
|4||Unglazed Ceramic Tile|
|Pocket Knife/Steel Screw|
|1-2||Cork Tile |
- Cork, vinyl floor tile and vinyl sheet flooring are Mohs 1-2. You can dent it with a fingernail.
- Site-finished wood flooring is Mohs 3 and can be scratched by a copper penny.
- Marble is also typically Mohs 4. It takes a steel screw or pocket knife to scratch it.
- Natural granite tops out at Mohs 5. Most can be scratched with window glass, but not with a pocket knife.
- Engineered quartz countertops and glazed ceramic tile are around Mohs 6 — tough stuff that can be scratched only with a case-hardened file.
If the company claims a Mohs of 8 or more, it's lying. No flooring is that hard.
Do not scratch a tile without the store clerk's permission. If the clerk does not give you permission, buy just that one tile, then see what will scratch it before committing to more.
You may not have to scratch it at all if the tile maker has conducted a formal Mohs test and printed a Mohs rating right on the box, and many tiles do.
The rating is stated as a number from 1 (softest) to 10 (hardest). Tiles with Mohs values of 4 or more are suitable for walls and backsplashes, 5 to 7 for most floors. Ratings of 8 9, and 10 do not exist. (If you find one, take a quick pic of the box and send it to us. We'd like to check it out.)
For very highly trafficked areas, or if hubby wears his golf shoes in the house, consider a Mohs value of 7.
These rule-of-thumb tests should not be relied on by themselves. Read the ratings on the box. But, you can use these to impress the sales clerk with your deep and comprehensive knowledge of fired clay tiles.
It also helps to throw around words like "monocottura" and "bisque" while you're at it, and refer often to ANSI A137.1 and wet and dry COF.
You'll have the guy on the run after just a few minutes.
Choosing the Right Tile for Your Application
So, now you know a lot of rules and a few little tricks about ceramic tile, but you are probably asking yourself how you can apply the rules to make the best decision about buying tile for your bathroom, kitchen or sunroom. Do you need PEI Group II or Group III, Vitreous or Semi-Vitreous? And, how about that Coefficient of Friction thingy?
Well, here's the table that shows how to put your new, in-depth tile knowledge to practical use in selecting the right tile for the particular use or application you have in mind.
When Do You Really Need Porcelain?
Porcelain is nuch too much tile for most applications, and just not worth the higher price for features you don't really need.
When do you actually need porcelain? Well, it depends on what you mean by porcelain, but here is a rough guide.
If You Mean Waterproof, "Impervious" Tile
For a residence, there is no indoor application that requires impervious tile. That's worth saying again: There is no indoor application that requires impervious tile. None. nada, zip, zero, zilch, bupkus.
Outside, it's a different story.
Outside the House
If you are using tile on a patio or deck in the Yukon (or Nebraska), you need a tile rated for frost resistance so it will withstand the repeated freeze/thaw cycles of northern winters without cracking.
Some tile rated "vitreous" in the water absorption test will stand an exterior environment in hard freeze areas — look for the frost rating.
A tile rated "impervious", will almost certainly be suitable for exterior use, frost rating or not. but it does not necessarily need to be called porcelain. Ignore the word "porcelain", look for "impervious" or the frost rating to be sure of what you are buying.
Inside the House
Inside the house, impervious porcelain is simply more tile than you need. If you examine the Application Guide, above, you will find that the best grade of tile you are likely to need is "vitreous" with a wear rating of Group IV for your floors in high traffic areas (hallways, kitchens, entries).
Some Basic Tile-buying Rules
1. There is no indoor residential application that actually requires impervious porcelain tile. Vitrious tile is more than adequate for any place where the tile will not freeze.
2. There is no glazed tile that needs to be full-bodied. Once the tile is installed, the body is hidden, so the body color is irrelevant.
If your tile seller says different, he is a big fat liar, liar, pants on fire.
Even this level of durability is more than is needed for low traffic floors, walls, backsplashes, and other surfaces that are not heavily walked on. Semi-Vitreous and Group III are likely more than enough.
If the tile is on a wall in an area other than a bath or kitchen, then most likely Non-Vitreous and Group I or Group II will work well enough, although, frankly, you will seldom find a tile rated in Group I in a tile store. You will have to go to a hobby or craft store to buy this grade.
You can, of course, buy impervious tile for use inside your house. Just expect to pay more for durability features you won't need.
It's like buying the car you need to get to work, to the store, and to church on Sunday (or Friday or Saturday, as the case may be).
You don't need a Ferrari. It would be nice to own one, and you can certainly buy a Ferrari if you don't mind the budget-walloping cost of a Ferrari, but you are not likely to use the extra features you are paying for like 220 m.p.h. on the flat or zero to 100 in 3.4 seconds. For what you are going to use it for, a Chevy is car enough.
The subway tile on the walls rates “Semi-Vitreous” on the ANSI water absorption test and the unglazed hexagonal mosaic floor rates “Vitreous” with a hardness rank in PEI Group II (light duty floors).
Both are called “porcelain” by their manufacturer following the European tradition which defines any light-bodied tile as a porcelain.
Neither would be porcelain under the American/ANSI definition of porcelain, but they are both completely adequate for their respective applications and will last a long, long time with reasonable care.
The same is true of ceramic tile. The most economical approach is to buy just the features you actually need.
Sure, tile that is rated Impervious may be harder and more durable and will last nearly forever — it's the Ferrari of tile. But, do you need a tile that will last nearly forever, or can you hobble along with one that will last a mere 500 years or so?
If a tile is rated for the application you intend, it will outlast you, your house, my 1968 VW microbus (maybe), and your greatgrandchildren's greatgrandchildren. Any extra durability is just a waste of money that could be used for something else. Upgrade your faucet, buy a better bathtub, or splurge on a romantic dinner for two at Vincenzo's.
Bear in mind that ceramic tile found in Pompeii has lasted over 2,000 years, survived at least one volcano and, by today's standards, is truly crappy tile.
If You Mean Light-Bodied Tile
If by "porcelain" you mean a light bodied tile, then whether you need it depends on whether the tile is glazed or unglazed.
Glazed light bodied tile has absolutely no place, purpose or function in the world. We have never in 40 years of remodeling found a use for a through- or light-bodied glazed tile. Not once. Not ever.
If the tile is glazed only the glaze will show once the tile is installed, so the color of the tile body makes no difference whatsoever to the appearance of the installed tile.
Of course, your friendly tile store clerk may recommend full-bodied tile "just in case" it chips or cracks. And, it is true that the damaged area will not be quite as obvious in a full-bodied tile. But, this is a false economy for at least two reasons.
First, the chance that a properly installed ceramic tile will chip or crack is extremely remote. Beating it with a hammer will do it, but not much else.
Second, the solution for a chipped or cracked tile is to fix it. None of the many tile resurfacing kits available on the market is perfect, but they do a better job for a lot less money than the extra cost of a roomful of through-bodied tile.
Better yet, replace it with one of the extra tiles you saved and stashed in the basement (attic, spare room, crawlspace, etc.).  (You did save some extra tiles, right? Right!?)
If the tile is not glazed, then it's a different story.
The color of the body is the color you will see, and if you need a light colored tile, traditional light-bodied porcelain may be your answer. If the size, color, and pattern are satisfactory just make sure the tile is rated for the the application you have in mind.
One common interior application that we see a lot of is mosaic floor tile which is often not glazed (to reduce slipperiness). This was a tile frequently used for bathroom floors in Victorian and Craftsman bathrooms — most often in bright white — so we use a lot of it in reproductions.
Light-bodied porcelain used to come primarily from Italy, and it was pricey. Nowadays it's made almost everywhere, and the price has dropped dramatically.
A tile that is rated for the application in which you intend to use it will outlast you, your house, my 1968 VW microbus (perhaps), and your great-grandchildren's great-grandchildren.
Any extra durability is just a waste of money.
The Bottom Line
So, here's the bottom line:
The Three Tile-Buying Do's and Don'ts
So, at the end of all this discussion, we end up with just three simple rules for buying fired clay tile.
Don't pay any attention to the word "porcelain" on a box of tile. The term is far too ambiguous with multiple meanings. It does not tell you anything particularly useful about the tile.
read the ratings on the box. Even if there are no handy icons, the ratings are probably in the fine print somewhere.
If there are none, pass it by. You do not want to buy unrated tile because you have no idea what you are getting. If you have questions about a rating or whether a tile will work for the application you have in mind, ask them of the manufacturer, not the clerk. The clerk probably knows less about it than you do after reading this article.
buy a better tile than you need. If you buy an impervious tile for your shower walls, you have probably paid more for features you don't need such as very high wear and frost resistance.
A semi-vitreous tile would work just as well, and most likely cost less. Of course, if you just found the tile sale of a lifetime, forget all this and splurge, splurge, splurge.
So, Is It Porcelain or Is It Ceramic?
We don't actually care, do we?
As we have seen, a tile deemed "porcelain" by its manufacturer is not necessarily better, harder, more durable, more scratch-resistant, or even necessarily more expensive than a ceramic tile; and if you paid more for a tile because it said "porcelain" on the box, you've just been had by a marketing scheme that dates back beyond the ancient Greeks to the even more ancient Chinese.
The only two things that actually matter are whether you like the look of the tile and whether it is rated for how you intend to use it.
So, if the tile salesman tries to "upgrade" you to a more expensive "porcelain" tile, look bored and yawn a lot. You know that there is no meaningful difference between ceramic and porcelain tiles.
Ah, what the hell, "accidentally" drop a box of tile on his toes for trying to trick you — deceivin' scoundrel that he is.