Wear and Tear
Some reasons why used Corelle or Pyrex may not look good as new. For all its legendary resistance to breakage, Corelle is not indestructible. It is only glass, and plenty of things can take their toll over the years.
Corelle, Suprema, Opal Pyrex and Corning Ware cups have excellent heat resistance and can withstand boiling water being poured into them. But prolonged usage of these cups to bring water to a boil in the microwave can cause pitting on the inside of the cup.
This can be visible as a loss of shine and it is unpleasantly rough to the touch as well. The damage can be thought of as an extreme version of dishwasher etching and the results are permanent. It can be mistaken for mineral build-up, but lime dissolving cleaners do not improve the situation since the surface of the glass is essentially eroded away.
"Cavitation" is the formation and collapse of low pressure pockets within a liquid. The reason cavitation erosion occurs is a complex physics problem, but to put it briefly, when a liquid is at a full rolling boil, each bubble is an area of low pressure and when the bubble collapses a shock wave is released onto the surface of the glass. It is a very hostile environment and each blast can cause miniscule bits of glass to flake away. Neither the temperature nor the microwave energy is the culprit in this case; it is solely due to the activity of the boiling water.
As for which glass type is more prone to this problem, it can definitely happen to Suprema, Opal Pyrex and clear Pyrex. So far it has not been observed on Corelle/Vitrelle cups, but there is no reason to believe it is immune. True Corning Ware cups are made of the same glass-ceramic as the cookware, and one would expect it is alright to use Corning Ware to bring water to the boil on the stove as well as in the microwave. But there are no guarantees either. Centura cups are unaffected because they do not go in the microwave.
It is possible to chip Corelle without shattering it completely. Chipping often occurs as a long thin sliver along the edge. The manufacturer advises not to use chipped Corelle. Chips can breach the compression layer, making the piece more fragile and vulnerable to breakage later.
Durable glass products like Corelle, Visions or Pyrex can appear to survive hard impacts or any flexing or prying actions as they occur. But the structure of the glass can be weakened in a way that is called "bruising". Bruising is invisible and can cause sudden shattering or cracking at a later date without provocation.
If an item has already been bruised, it is more likely to break apart when it is wet, and the temperature of the water is unimportant because this type of breakage is unrelated to thermal shock. Water has a way of seeping into even the smallest crack, effectively prying it apart at the molecular level. The phenomenon is known as crack propagation. Bruised glass can also break while dry, and even trivial temperature changes can sometimes be blamed.
With careful handling, bruising can be avoided. When nesting pieces together it is important that they do not jam together at awkward angles. Avoid nesting differently shaped items that were not designed to go together, particularly circumstances that create pressure points. Once bruising has occurred it is impossible to detect, and it is also impossible to avoid the situations that hasten the onset of shattering. If it is going to break, it will.
Little rough chips can form on the edges of Corelle, as if the top layer of glass is flaking away. This is usually caused by automatic dishwashers that are too vigorous and cause the dishes to jiggle with every rotation of the spray arms. It is entirely possible that edge wear can occur in homes with no dishwasher at all after a few decades of daily use. Sometimes the edge can be worn in a way that it feels like it has been sanded down and lost its shine.
In any case, the damage is unsightly and the rough exposed area is prone to permanent staining. To check quickly for roughness, push the tip of a fingernail around the very edge. It should feel slick and smooth, and look shiny too.
Grey Metal Marks:
Grey marks result from rubbing or scraping metal cutlery on the dish during use, and steak knives are a particular culprit. The marks can also appear on edges and on the outside of cups and bowls. Rubbing up against stainless steel sinks is the reason for grey marks on the exterior. The marks might come off, or they might not. The lighter the mark appears, the more likely it will come off with the appropriate cleanser*. If the grey mark is within a deep scratch or abrasion it can be very difficult to remove it.
Corelle is ideal for microwave use, but in rare circumstances the bottom of the dish can drop off after microwaving. Very often the hole left in the dish is the same shape as the hot food that used to be there, which is not a coincidence.
The main reason for fractures is that the food is too hot too fast, creating a severe temperature difference between the glass under the hot food and the rest of the dish. Since the glass itself does not get hot in the microwave, the part not touching the food will stay relatively cool, especially during a short cooking time.
Certain situations will make fractures more likely, and should be avoided: a small amount of food on a dish that is too large, starting with a cold dish from the refrigerator or freezer, or food with high fat and/or sugar content that heats up especially fast. Small amounts of food should be heated slowly on a lower power to avoid dropping hot bottoms.
Rust Spots, Yellowing:
There is no metal content in Corelle, so any brown/orange rust spots that appear on the dishes is always caused by contact with something else that is rusty. Most commonly, rust on the edges is caused by contact with rusty dishwasher racks. The spots often can be removed with the appropriate cleanser*, but if an item has edge wear as well as edge rust, the marks might be permanent because the porous worn edge will not give up stains easily.
Water with a high iron content can produce an all-over yellow or orange build-up after years of washing. Usually this can be removed successfully with the appropriate cleanser*.
Spontaneous brown shiny residue:
Decorated opal Pyrex is sometimes vulnerable to a peculiar type of oxidation or chemical reaction developing in a random mottled pattern on the coloured exterior. Items that were put away in perfectly clean condition can acquire strange brown patches after a period of months or years. It can even appear on Pyrex that has remained sealed in its original box. The residue has a slight metallic shine, and if it is thick enough it can seem like an accidental application of gold spray paint.
This 024 casserole spontaneously developed brown shiny residue after it was washed and put away. The marks were successfully removed.
Washing with soap & water helps to get rid of the residue, but an appropriate cleanser* might be required to completely remove all traces. But because some colours soak up stains more readily, a faint brown discolouration might be left behind in some cases.
A certain number of scratches are to be expected unless the piece has never been used at all. To evaluate how much scratching is present, study the item under a bright light, held at an angle where the light is visibly reflected on the surface. Swirly scratches in the bottoms of cups are common, and some plates can be so thoroughly used that the centre might have no shine left at all. Another area that is vulnerable is the "speed bump" on saucers if they have seen frequent contact with cutlery.
Yellow Sugar Bowl Lids:
The most common misfortune of sugar bowl lids is that they are missing completely, but when they do survive, they are almost always yellowed with age. There are two kinds of hard plastic that lids are made of, one is solidly opaque, and the other has a pearly translucence. Opaque plastic lids are meant for Corelle hook-handle sets as well as Expressions sets made of Centura. The translucent type goes with the large Pyrex set. Style wise, the Expressions sugar lid is the same shape as the one for the Pyrex set, but it is the same type of plastic as the hook-handle lid.
Sugar bowls (L-R): Corelle "hook-handle", Expressions, Pyrex, Corelle "hook-handle". The two lids on the left have yellowed. They are opaque plastic like the white lid, bottom right. Top right is the translucent kind; it is still white also.
Both types of plastic can suffer from yellowing to varying degrees. Exposure to ultraviolet light, either from daylight or fluorescent lighting, can be the primary cause of the colour change. Heat can have a yellowing effect also, and acids, oils and pigments from food can cause stains too. Cream & sugar sets that have remained unused in the original box often have pristine white lids, but they will turn yellow too if they are not continually protected in a clean, cool and dark place.
The original small Pyrex sugar bowls came with clear glass lids. Pyrex margarine dishes also have plastic lids, but they are a soft type of plastic that seems less prone to yellowing.
* -What is an "appropriate cleanser", and how to tell if it is working:
Many diverse cleansers might work, but some can destroy the colour and lustre of enamels, glass and glass-ceramics. The manufacturer recommends CorningWare Cleaner & Conditioner, which first became available in 1985. It is a mild abrasive and solvent with a thick creamy consistency, and it works well on rust spots and grey marks on Corelle, Corning Ware, Centura and opal Pyrex.
Grey marks on the glass or glass-ceramic itself can be extremely stubborn, so the careful use of a stronger abrasive can make their removal easier. A powdered cleanser like Barkeeper's Friend is a good choice for this purpose, but it is capable of leaving scratches. On the fragile coloured exterior of opal Pyrex, Barkeeper's Friend is too abrasive and can easily ruin the shine.
CorningWare Cleaner is a better choice on opal Pyrex colours, but if grey marks are being difficult, excessive scrubbing can also dull the shine. A small test patch should evaluate whether the product will succeed on a certain colour.
Using a liberal amount of cleanser, grey marks and rust stains should begin to melt away with a few strokes. This means the enamel is "hard" enough not to have soaked up the stain and it will stand up to the cleanser too. If there is no visible progress after a few rubs, then the product is not going to work very well in this case; the coloured surface might be "soft" and vulnerable to damage from more persistent scrubbing.
Colours that clean up easily are often newer Pyrex patterns, but some of the earliest Pyrex colours turn out well too, so the age of the enamel is less important than the colour. Removing grey marks and brown stains from shades of red and pink is sometimes impossible for some reason.
What are Glass-Ceramics?
What is Suprema?
What is Centura?
What is Vitrelle?