The difference between glazed and unglazed porcelain tiles

Today’s ceramic tile industry seems to have almost as many ‘types’ of tile as it has ‘styles’ of tile. We have certainly come a long way since the 1970s when most bathroom wall tiles were a 6” square Johnsons “chicken” tile, as it came to be known, in half a dozen different colours, with glazed mosaics from Japan on the floor.

Nowadays, as well as the bicottura and monocottura types that dominated the residential market until the late 1990s, we have vitrified porcelain, both full-bodied and double-loaded, and a plethora of glazed porcelain tiles, available with a matte glaze, a gloss glaze, a ‘lappato’ a.k.a. semi-polished finish, plus a variety of textured, slip-resistant tiles. Likewise, unglazed, fully vitrified porcelain tiles come in a variety of finishes. It is not always easy to tell what sort of tile it is just by looking at it, but knowing the difference is important in order to choose the right tile for the situation, and to avoid mishaps and accidents. 

To assist you in sorting your way through this maze of confusing terminology we have put together a helpful list of the most common types of tile and their performance characteristics, where to use them, and how to maintain them. We also correct some misconceptions and misleading information regarding glazed and unglazed tiles.


Ceramic tile: Is formed of clay, plus sand, felspar, quartz and water, which is then dried to a dust, pressed at great pressure and fired at temperatures ranging from 1000 to 1300 degrees Celsius, depending on the type of tile. Ceramic tiles can be produced using either the MONOCOTTURA or BICOTTURA method. The monocottura (from the Italian meaning “single fired”) method is generally used to produce floor tiles, though they can be used on the wall if desired. With monocottura tiles the body and the glaze are rapidly fired at high temperature at the same time. Until the late 1990s the vast majority of all residential floor tiles were made using this method.

Collage of various ceramic types - monocottura, bicottura plain, bicottura patterned, matte unglazed porcelain & glazed porcelain.

Wall tiles, which have a wider range of colours and decorative surfaces, are produced using the bicottura method. Although the translation implies these tiles are fired twice, they may in fact be fired several times, depending on the number of layers of glaze applied. The firing temperature of bicottura tiles is lower than for monocottura tiles, as hardness is not the required result. Generally speaking, the higher the temperature the harder the tile. 

Ceramic tiles have a water absorption rate of over 4%. The fact that they aren’t fully vitrified should not be seen as a fault or a limitation, as it is an unnecessary process in their manufacture.

Porcelain tile: Is actually part of the ceramic family, but distinct in its characteristics due to the types and mix of clays used, and the specific methods of production. Porcelain is a much stronger version of ceramic. Both types are made of clay and are fired in a kiln. The main differences are that porcelain tiles have a water absorption rate of less than 0.5%, a result which classifies them as fully vitrified, making them incredibly hard and rendering them suitable for heavy traffic floor use, including industrial situations. Porcelain tiles can be used in virtually any situation - from light traffic bathroom floors (and walls) to factory floors and swimming pools, both in (as mosaics) and around them. 

This porosity rate is achieved due to three main factors: the incredible force used to press the clay - up to 100,000 lbs per square inch; the volcanic temperature at which it is fired - approx. 1250 degrees Celsius; and the mix of clays and minerals used. It is the difference in the clay mix - kaolin, felspar, quartziferous sand & metallic oxides, which is at the heart of all the other factors, for it is the clay which enables the extreme pressing and firing to occur. (Kaolin, a white china clay, is crucial in aiding the tile to maintain its shape during the firing process.) By injecting the pigmented dry dust clay into moulds and pressing at this incredible pressure the clay dust particles are compressed very close together, reducing the amount of air and moisture between the particles. This makes for a denser, less porous body. The nature of the porcelain clay is such that when it is fired at approx. 1250 degrees Celsius the dust particles melt and fuse together, resulting in vitrification.

Unglazed, vitrified porcelain with ‘full body’ colouring.

Vitrification: Is a process. It is the transformation of melted clay into a ‘glass-like’ substance with a porosity of <0.5%. The greater the vitrification the harder the tile and the less porous it is, meaning the less likely it is to absorb liquids and be stained, and the harder the surface the less likely it is to scratch or chip. Of course, the less likely moisture is to penetrate the surface of a tile the more likely it is to sit on the surface, which can, depending on the texture of the surface, increase the potential for slipping. 

There are four types of vitrified tile: full body, double loaded, glazed & soluble salt. For several reasons this last type is not something that will be considered in this article.

Unglazed Porcelain:

Full-bodied porcelain is a type of vitrified tile where the colour runs right through the body with a single colour from the surface to the base of the tile. Not only is the colour consistent throughout, but the technical properties, such as water absorption, frost & acid resistance are also consistent.

Fully vitrified, unglazed porcelain tessellated tiles with a matte finish being laid on an external pathway. Note, the colour is consistent all through the tile body.

Double-loaded tiles are made by compressing two layers of clay, generally of different colour, together before firing. As the clay is porcelain throughout, the two layers fuse together producing a completely homogeneous product. This is usually done when the colour of the top layer is produced using very expensive pigments, e.g. cobalt. By restricting the colour to the top 3-4mm instead of the total 9-10mm, it keeps the costs under control. In the case of encaustic tiles made in the traditional way, when the base tile is pressed, the decorative pattern is stamped into the top surface resulting in recessed areas which are then filled with contrasting coloured ‘slip’ (liquid clay) and then fired to produce a homogeneous porcelain tile, with the same properties as a conventional full-bodied porcelain product.

Double loaded unglazed vitrified porcelain - terrazzo look.
Winckelmans fully vitrified porcelain showing penetration of different colours below the surface of the tile.
A bathroom floor using an example of a double loaded porcelain tile - Winckelmans fully vitrified, unglazed porcelain encaustics.

Polished Unglazed Porcelain: Polished porcelain is shiny but it does not have a gloss glaze on it. It is an unglazed tile that has been mechanically polished in the same way that marble or granite tiles are polished to make them shiny, and just as polished marble or granite tiles need to be sealed to prevent them from being stained, so does polished porcelain. 

When porcelain is polished its technical characteristics are altered, especially its absorption level. Because of its finely textured surface, standard, or ‘matte’ porcelain does not absorb dirt and stains, but can exhibit soiling which is confined to the surface of the tile and can be easily washed off. Mechanical polishing by the factory creates a smoother surface, which is much less likely to grab and hold dirt from shoes, etc, but damages the micro beads of porcelain dust and opens up the micro pores of the tile rendering the surface more porous and susceptible to staining, necessitating the use of a penetrating sealer. Polishing, which grinds the surface of the porcelain smooth - to a shiny, mirror finish, also results in the tile becoming slippery when wet. (Shopping malls tend to prefer polished porcelain tiles for their public thoroughfares because they are easier & cheaper to clean, despite the fact that they are more slippery, i.e. dangerous, when wet.) Once a penetrating sealer has been applied to the surface of the polished, or lappato, unglazed porcelain tiles it then becomes necessary to clean them using a specially formulated pH neutral cleaner. Standard floor cleaners have a high pH level, which breaks down the sealer, shortening its life and reducing its efficacy.

Applying a quality penetrating sealer will overcome the problem of staining, but not the issue of the tile’s slipperiness, therefore it is recommended that polished porcelain be used on floors where there is little or no chance of them getting wet, or on the wall. Matte finished tiles are slip resistant, but it is always necessary to check the slip rating with the manufacturer. Even with matte finished unglazed porcelain, different colours may yield a different slip rating. Unglazed porcelain with a lappato, or honed finish, will have a slip rating somewhere in between that of the polished and the matte. Textured surfaces have a very high non-slip rating, but are hard to clean; their use is recommended for external areas only.   

Glazed Porcelain: Most people assume the word glazed means ‘shiny’ or ‘glossy’. However, in the tile industry the word is used to refer to the coloured or patterned coating that is applied to the surface of a tile. Glaze is a substance rather than a description of a textural finish, e.g. one can have marble that has a ‘glossy’ surface or  unglazed porcelain that is ‘glossy’, but neither products are ‘glazed’. Think of glaze like paint - just as you can have a matte paint for your walls, and a gloss paint for your doors and skirting. Glaze is paint for your tiles. 

Matte finished glazed porcelain decorative wall/floor tile.
(c) Beecraft Kitchen splashback features decorative glazed porcelain wall/floor tiles. To clean, just wipe down with warm soapy water.

Virtually all wall tiles, and a great many floor tiles, are glazed. Glaze is applied before the tile enters the kiln; it then fuses to the surface of the porcelain body during the firing process. Just as with unglazed porcelain tiles, glazed porcelain has four basic glaze finishes: gloss, matte, lappato & textured, of which there are several different types. Tiles with a lappato glaze have a higher shine than matte tiles, but a gentler finish than a gloss tile. This gentle polishing process takes its name from the Italian word for “semi-polished’ or ‘honed’ - honed being a common expression in the stone industry for marble with a satin, rather than a highly polished, shiny surface. 

Inkjet Printing: Recent advances in glaze technology have made it possible to print digital images of anything one can imagine onto porcelain tiles using inkjet printing machines. Inkjet machines have up to 1000 nozzles and work with six different colours at up to 1000 dpi resolution. The most important feature of this printing process is the amazing versatility: absolute accuracy of images; elimination of screens, which resulted in a pixelated effect; excellent potential to reproduce natural stone, timber, terrazzo, concrete or any pictorial image at all; and the ability to print to the tile’s edge. This accounts for the popularity of glazed porcelain tiles being used for walls as well as floors in bathrooms, with matte glazed tiles being more commonly used on the floor and gloss or lappato glazed tiles on the walls. Another unique feature of this method is the ability to print on a non-flat surface - even curved or textured surfaces, which opens up the scope of what type of image can be produced and where the tiles can be used. Only by inkjet is it possible to lay down decoration on top of frit grains, because it is done without contact with the surface.

Inkjet printed glazed vitrified porcelain - limed timber look.

Frit: Frit is a ceramic glass that has been premixed from raw powdered minerals and then melted, cooled and then ground into a fine powder. The type and quality of frit will affect the durability of the glaze, and raise or lower the hardness, colour & gloss level of the glaze. 

Glazed porcelain tiles do not require sealing - that is the purpose of the glaze. Think of them like your ceramic dinnerware; your plates don’t need sealing to protect them from staining, but they do need washing.

One of the aspects of glazed porcelain that is often seen as a problem, or a potential problem, concerns damage to the glaze. What are the practical and aesthetic consequences of a tile’s glazed surface being damaged? The answer to this question has more to do with the quality of the glazed porcelain tile and where it is being used than it does with glazed porcelain tiles per se. If the quality of the tile is high then the likelihood of scratching or chipping the glaze by dropping something on it is extremely low. Remember, these tiles are manufactured to be laid on the floor and walked on. Some glazed porcelain tiles have a porcelain body that is coloured to match or closely resemble the colour of the glaze, should damage to the surface occur. If there is a significant difference in the glaze colour and the body colour it will show up if the surface is damaged, so it is advisable to avoid using such a tile in a heavy traffic area.

Matte glazed porcelain with colour matched porcelain body.

Abrasion Resistance: This is the capacity of the glazed surface to resist the wear caused by foot traffic or the abrasion caused by mechanical equipment, and is classified into 5 categories, depending on the areas the tiles are to be used in:

PEI I - Tiles for areas with light traffic, not exposed to abrasive dirt, e.g. bedrooms, en-suites, guest bathrooms.

PEI II - Tiles for areas with average traffic & medium to low abrasion, e.g. living/dining rooms, powder rooms.

PEI III - Tiles for areas with high/medium to high traffic & average abrasion, e.g. entrance, hallways, residential kitchens, family rooms.   

PEI IV - Tiles for areas with intense traffic, e.g. restaurants, bars, cafes, offices, shops.

PEI V - Tiles for areas with especially intense traffic, e.g. shopping malls, schools & universities, airports, factories.

Misconceptions about vitrified & glazed porcelain tiles

In researching this article I have come across an enormous amount of false and poorly researched and written material, which only adds to the confusion of people looking for accurate, helpful information. You may have read or been told certain things that are asserted about the supposed ‘negative’ aspects of vitrified porcelain tiles and the reasons to avoid using them. Let us go through these assertions to clarify the situation.

  1. Vitrified tiles are slippery when wet.

    Most tiles have the potential to be slippery when wet, whether they are glazed or unglazed, but matte finished tiles are far less so, and textured or grip finished tiles are non-slip, as they are specifically manufactured for external use. If your foot is dry and the floor is dry, you will not slip. If your foot is wet and the floor is wet you will not slip. For a slip to occur, one surface needs to be wet and one dry, which is the case when coming out of the shower cubicle, which is why, regardless of the tile on the floor, we step onto a bath mat.
  2. Vitrified tiles are difficult to install.

    No explanation is given to justify this claim, so it is hard to know what is being referred to, but vitrified porcelain tiles are no more difficult to install than marble tiles or ceramic wall tiles. Porcelain tiles are extremely strong and hard, which will necessitate in some situations the use of a water-cooled bench saw to cut. These days, porcelain tiles can be produced in very large sizes, including 450x900mm, 900x900mm, 600x1200mm, etc. These sizes will need to be cut on a bench saw, but the widespread use of large format tiles would suggest that there is a market for such tiles. Obviously a 900x900mm tile will require two men to lay it, as much because of its weight as its size, but the same could be said for 600x600mm marble tiles.
  3. To ensure stability, high quality adhesives need to be used.

    I’m not sure how the use of “high quality” adhesives is seen as a negative. Surely you want your floor and wall tiles to stay in place for as long as possible? The longer they stay in place, the less the installation costs you when amortised over the life of the tiled area. This assertion also implies that poor quality adhesives are sufficient when installing all other types of tile.
  4. Vitrified tiles need to be installed by an expert tile layer to ensure thin, even and neat joints.

    Again, I’m not sure how engaging a skilled tiler is viewed as a negative feature specific to vitrified tiles. Replace the word “vitrified” with “ceramic”, and the same could legitimately be said: “Ceramic tiles need to be installed by an expert tile layer to ensure thin, even and neat joints.” In fact, the need for neat joints is more necessary for wall tiles which, in most cases, are bicottura ceramic.
  5. Defective or damaged vitrified tiles must be cut away along the grout line, and surrounding tiles could be damaged in the process.

    There are a few points here that need addressing. This scenario is potentially true for any tile, no matter what it is made of - monocottura ceramic, glazed or unglazed porcelain, marble or granite. If the grout is removed first, the chances of breaking any surrounding tiles is greatly reduced, however, the softer the tile the more likely it is to be damaged during this process. Fully vitrified unglazed porcelain tiles are extremely strong and resistant to scratches and chips, therefore the likelihood of them needing to be replaced is very low, and because the colour goes all the way through them, chips and scratches don’t show up. In the case of glazed porcelain, where the glaze colour may be different to the vitrified porcelain body, chips will be apparent, but the glaze on a quality porcelain tile is very hard. After all, these tiles are intended to be walked on and are designed and manufactured accordingly. If they are so hard that they need a diamond blade saw to cut them, then they are going to cope with wear-and-tear. The glaze on a quality porcelain tile will generally have a MOHS hardness of 8, which is greater than most marbles (MOHS 3-4) and as strong as some igneous rocks, e.g. granite and quartz. The maximum on the MOHS Scale is 10, which is the hardness of diamond.
  6. Every year or two the grout must be resealed or it will begin to look dirty. The sealant will create a hazy finish on the tiles if allowed to dry.

    Surely this is a grout issue or a sealer issue, not a tile issue? Nevertheless it is misleading. When the floor is washed the grout gets washed too. There are additives that can be put in the grout when it is being prepared to prevent mould. If a top quality sealer is used then it only needs to be done once, or at the worst, every 15 years or so. If the sealer is applied with a thin brush it will not get all over the tiles, and if any excess is wiped off before it dries it will not leave a “hazy finish”; and if none of the above things are done you will be left with a “hazy finish” regardless of the tile you use.
  7. It is unlikely that you will find the same tile after some years and hence, when you buy tiles, you should buy a few extra tiles for future repairs.

    This is actually sound advice, but is true for ALL tile purchases, not specifically vitrified tiles. It is certainly not a reason for not using vitrified porcelain tiles. When purchasing tiles - any sort - the rule of thumb is to allow a 10% wastage factor, the emphasis being on ‘wastage factor’. When the job is completed you may not have any tiles left, however having some spares is the cheapest insurance policy around. The reasons for needing extra tiles are myriad, but often are the result of a plumbing issue or rearranging the layout of the kitchen, so one extra tile will not be sufficient. Remember, the larger the tile the greater should be the wastage factor - your tiler only needs to break one tile and you may have lost between half and a full metre. If you are using Winckelmans 100x100mm vitrified porcelain tiles, not only are they extremely strong, the colour goes all the way through the body, and there are 100 pieces to the sq. metre, not one or two. Also, Winckelmans have been making the same colours and shapes for over a century.
  8. Vitrified tiles are not environmentally friendly, as the process of making them causes significant cost of energy and carbon dioxide.

    On the basis of this argument we should also steer clear of cars, cows and aeroplanes. If it was true, which it is not. It has been standard procedure in Europe amongst reputable tile manufacturers to implement rigorous green policies to do with their manufacture. European ceramic and porcelain manufacturers use 100% plentiful, natural materials in a commitment to zero-waste manufacturing processes. Italian, Spanish and French porcelain tile producers are governed by some of the most stringent environmental legislation anywhere in the world. Legislative requirements control raw material utilization, anti-pollution production devices, post-industrial recycling, air quality, and the use of recycled materials for product packaging. The result is manufacturing technologies with a lower environmental impact, and products that deliver excellent ecological qualities. European environmental standards for the porcelain industry are certified through ISO 14001, EMAS and the European Union Ecolabel. At the other end of the supply chain, porcelain tiles offer energy efficiency with inherent thermal mass that reduces peak heating and cooling to moderate temperature in the home or workplace. Also, due to the sourcing of materials, ceramic and porcelain tile has the lowest carbon footprint of any flooring choice.
  9. Vitrified tiles are more expensive.

    This assertion is rarely if ever qualified. “More expensive” than what? European glazed and unglazed porcelain tiles may be more expensive than porcelain products from non-European countries, but are you comparing apples with apples? In light of the environmental considerations outlined in #8 above, it is clear to see why the European ceramic product comes with a premium price when compared to the tiles from countries where environmental considerations play no part. However, the E.U’s Life-Cycle Assessment (LCA) of porcelain and ceramic tile as a finishing material ranks them the highest for expected life and lowest for cost per sq. metre per year amongst ‘all’ floor covering types. And now, with digital printing being able to exactly replicate the look and feel of marble, travertine, limestone, terrazzo, concrete, metal and myriad different timbers, the issue of price comparison takes on a completely new aspect, as one compares not just the initial cost of, say, real Carrara marble tiles with the glazed porcelain lookalike, but the shipping and installation costs, the cost of the special adhesives needed for stone compared to that for porcelain, the cost of sealer and sealing plus the special pH neutral cleaners. So, in response to the initial assertion, it is essential to ask, “Compared to what?” and, “Are you comparing apples with apples?”
Winckelmans technical specifications.


The above article is an attempt to help sort through, and make sense of, the vast array  of competing types of ceramic and porcelain tiles, and the confusing and sometimes misleading claims for one type over another. 

All porcelains, whether glazed or unglazed, have legitimacy when used in the correct way in the appropriate situation. A Ferrari is a great car, but if you want to use it to plow a field or drive through the bush it will fail to achieve what is being asked of it; a case of ‘Horses for courses’.

Winckelmans unglazed vitrified porcelain can be used in an unlimited range of situations, but it has certain limits which are aesthetic rather than technical. Those aesthetic limits have to do with size and the absence of glazed, ‘faux’ finishes. But what the Winckelmans ranges offer professional specifiers as well as end-users is the ability to create unique designs using a vast spectrum of colours that coordinate with large format, inkjet glazed porcelain or handmade subways. The only real limit is your imagination.