Wear and Tear

Some reasons why vintage Corelle may not look good as new.

For all its legendary resistance to breakage, Corelle is not indestructible.  It is only glass after all, and plenty of things can take its toll over the years.

Cavitation Erosion

Corelle, Suprema, Opal Pyrex and Corning Ware cups have excellent heat resistance and can withstand boiling water being poured into them.  However, 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 perhaps for mineral build-up, but lime dissolving cleaners do not improve the situation since the surface of the glass is essentially eroded away.  In a small way, an abrasive cleanser* vigorously applied can take the edge off the roughness.

The reason the damage 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 and Opal Pyrex (and clear Pyrex too).  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 do not enter into the issue since they do not go in the microwave at all.

(Cavitation refers to the formation and collapse of low pressure pockets within a liquid.)

Chips:

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.

Delayed Shattering:

Durable glass products like Corelle (and Visions & Pyrex too) can appear to survive violent knocks or any flexing or prying actions as they occur.  However, the structure of the glass can be weakened imperceptibly in a way that is technically called ‘bruising'.  This can cause sudden shattering at a later date with or without any provocation.  Less spectacularly, bruising can also cause a dish to crack spontaneously in half, or a chunk to pop unexpectedly off the edge.

Edge Roughness:

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 too 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 porous exposed area is prone to staining.  To check quickly for roughness when purchasing vintage Corelle, run a fingernail around the very edge.  It should feel slick and smooth to the touch, and look shiny as well.  There is no cure for rough edges.

Grey Metal Marks:

These are the result of rubbing or scraping metal utensils on the dish during use, and steak knives are a particular culprit.  The marks can also appear on the edges of Corelle and the exterior of Pyrex bowls, and stainless steel sinks are the prime cause of grey marks on these areas.  They 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, for example from a knife, it can be very difficult to remove the mark.

Heat Fractures:

Corelle is perfect for microwave use, but the phenomenon of the bottom dropping off once the dish is taken from the microwave happens from time to time.  Very often the hole left in the dish is the same shape as the hot food that used to be there.  This is not a coincidence.  The main reason this happens is the food is too hot too fast, creating a dangerous 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 of the dish not touching the food will stay relatively cool, especially during a short cooking time.  Certain circumstances 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 can be heated more slowly on a low power to avoid dropping hot bottoms.

Rust Spots:

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's rusty.  Very commonly, rust on the edges is caused by contact with rusty dishwasher racks.  The spots often can be removed with the appropriate cleanser*.  But it's entirely possible too that they won't come off.  This is especially true if the piece has edge wear as well as edge rust; the porous worn edge will not give up stains easily.  Water with a high iron content can produce an all-over rusty build-up after years of washing.  Usually this can be removed successfully with the appropriate cleanser.

Surface Scratches:

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 dish under a bright light, held at an angle where you can see the light reflected in the surface.  Nothing can be done about the scratches, but that's how you can find them anyway.  Swirly scratches in the bottoms of cups are particularly common, and some plates can be so thoroughly used that there are areas that have no shine left at all.  Another area that is quite prone to wear is the ‘speed bump' on saucers if they have seen frequent contact with cutlery.

Yellow Sugar Bowl Lids:

The most common misfortune to befall sugar bowl lids is that they are missing completely.  When they do survive, they are almost always yellowed with age.  There are 2 kinds of hard plastic that these lids are made of, one has a solid opaque appearance and the other has a pearly translucence.  The opaque plastic is used with Corelle hook-handle sets as well as the Expressions Centura sets.  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.

The 2 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 too will turn yellow if not continually protected in a clean, cool and dark place.

(The original small Pyrex sugar bowls had clear glass lids.  Pyrex margarine dishes also have plastic lids, but they are a softer, flexible material and seem to be less prone to yellowing.)

* -What is an 'appropriate cleanser'?

Many diverse cleaning products might work very well, but some products can destroy the colour and lustre of Corelle, Pyrex and Corning Ware.  The manufacturer recommends 'Corningware Cleaner & Conditioner'.  It is a mild abrasive with a thick creamy consistency, and it works well on rust spots and grey marks on Corelle, Corning Ware and the opal unfinished surface of Pyrex.  But it is most important to be extremely careful with the coloured exterior of vintage Pyrex.  The finish can be fragile, and on older pieces this product may simply remove the shine without removing the stain.  A small test patch might be wise before undertaking a full scale application.