As manufacturing of unglazed porcelain moves out of the traditional strongholds of Italy and Spain and into the greater world economy, porcelain from other regions (such as Asia and South America) is being branded inferior. The main yardstick used to measure this inferiority is porosity. Problems such as staining and difficulty in maintenance are attributed to these inferior products because they are more porous. BUT ARE THEY?
It is this general view that poor quality in porcelain is due to low porosity that needs to be scrutinized and clarified because it is in many cases simply confusion between porosity and pores.
So what is the difference between porosity and pores? Porosity is by definition the ratio of voids to solids in a tile. It is measured by the amount of water a tile can hold expressed as a percentage of it’s own weight. The test in simplified form is carried out by firstly weighing a dry piece of tile, soaking it for 24 hours in water – drying the surface water – and then weighing the wet piece. The tiles porosity is the difference in weight between the dry and wet piece. It is expressed as a percentage gain over the dry weight and indicates how much water or moisture a tile can absorb.
A pore is something different. A pore by definition is a hole or opening on the outer surface of an animate or inanimate object. Surface pores are not measured in a quantifiable manner and hence are not part of the ceramic or porcelain standard. However they do have a large influence on how porcelain will perform. Moreover it is a tile’s pores rather than just porosity in isolation that create many of the cleaning problems and hence should be scrutinized when deciding if a product is high or low quality.
Cleaning & Staining of Porcelain
The problems most associated with “poor quality” porcelain are most always related to cleaning and maintenance. Common problems are the porcelain will stain. It will mark (especially from car tyres) and soil easily. It will stain during the grouting installation especially when grouts with high contents of coloured oxide are used. The reason for this sub-standard performance? Porosity. The porcelain has high porosity and hence absorbs these contaminants. It is therefore poor quality.
Unfortunately this conclusion is only part of the truth. Porosity is certainly one of the main factors in creating these problems and in determining quality. However it is the surface pores that play an almost equal part. If a porcelain has large surface pores (these are still very small mostly visible through a microscope) then it will suffer from exactly the same cleaning and maintenance problems. The pores do two things. They increase the surface co-efficient of friction thus doing a better job of collecting dirt (cleaning the bottom of your shoe). Secondly they provide below surface reservoirs to hold the dirt. The end result is that the pores create exactly the same problems as porosity and is the reason why so many people confuse the two characteristics.
Quality equals Porosity & Pores?
The quality of a porcelain product is determined principally by the blend of clay that is used and fired. The higher the content and purity of white Porcelain Clay (China Clay) the better the porcelain. Our two characteristics, porosity and pores, are directly related to the China Clay. With high quality clay the tile has less volatiles and hydrocarbons to burn off during firing. This results in a tile that is denser and hence low porosity. However it also results in a tile that is harder with better vitrification. This means that there are less surface pores. This becomes even more important for the polished porcelain. The polishing process has a habit of creating pores as the grinding/polishing process removes softer particles leaving behind surface holes or pores. The harder the tile and the more even the body composition the less pores are created. So it is both the porosity and the amount and size of surface pores that effects performance and helps to characterize quality.
Most problem solving for these “inferior” porcelains is done by way of sealing. A penetrating sealer is applied with the expectation that this will lower the offending porosity. However it will have no positive effect on the surface pores. In other words the traditional diagnosis of porosity as the only cause will lead to the wrong solution in many cases.
To protect and ease maintenance of porcelain with excessive porosity a penetrating type sealer is the correct solution. However to effectively protect and maintain a porcelain with excessive pores a combination of a coating pre-grout sealer and a heavy duty cleaner is required. Porcelain with large pores will cause most problems during the grouting process. The grout gets caught in the micro pores, hydrates and bonds making it impossible to clean without using harsh acids. To solve this problem the porcelain should be pre-grout sealed with a water base, water strippable protectant. This will stop the grout filling the pores. Once grouted the pre-grout sealer is removed and sealed with a penetrating sealer if it also has high porosity. If it doesn’t then no sealer needs to be applied.
Identifying if porcelain suffers from high porosity or excessive pores is sometimes difficult. However the easiest way is to carry out a simple stain test. Expose a sample for 24 hours to a typical household contaminant such as red wine or cooking oil. Apply (preferably a contrasting colour) a cement grout to a second piece and try to clean.
If a tile suffers from high porosity it will be stained by the household contaminant. If the surface pores are the problem then it will be the grout that creates the problem. There will be of course those porcelains that stain from both sources. In these cases a pre-grout coating sealer should be applied during installation and a penetrating type applied after.
It is understandable that porosity is so often blamed for the cleaning and maintenance issues surrounding some porcelain. Likewise it is equally understandable that it is associated with inferior quality, because high porosity and surface pores both create the same problems. However in many cases the pores create these problems. An understanding of the difference is imperative so that the correct solution can be implemented. It is also important to acknowledge that an inferior porcelain is not one that simply has high porosity. The number and size of surface pores is equally important. Moreover there are many porcelains (mainly polished) that have low porosity but suffer the cleaning problems due to their pore characteristics. Of course there are other factors such as size, selection and kiln contamination etc that help determine quality. However it is the porosity and pores that can most easily be modified after manufacturing to ease problems that makes them perhaps the most important to understand.