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.

Cavitation refers to 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 do not enter into the issue since they do not go in the microwave at all.

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.  Chips can breach the compression layer, making the piece more fragile and vulnerable to breakage later.

Delayed Shattering:

Durable glass products like Corelle, Visions and 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 imperceptibly in a way that is technically called 'bruising'.  This can cause sudden shattering at a later date without any obvious provocation.  Less spectacularly, bruising can also cause a dish to crack spontaneously in half, or a chunk to pop off the edge. 
 
If a piece has already sustained bruising it is more likely to break apart when exposed to water, and the temperature of the water is unimportant as this type of breakage is not due to thermal shock.  Water has a way of seeping into even the smallest crack, effectively prying it apart at the molecular level.  This 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 which create pressure points.  Once bruising has occurred it is impossible to detect, and it is also impossible to avoid the situations which hasten the onset of shattering.  If it's going to break, it will.

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 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 permanent staining.  To check quickly for roughness when purchasing used 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, on the exterior of cups and Pyrex bowls, and on the handles of Corning Ware.  Stainless steel sinks, not utensils, are the prime cause of grey marks on these areas.  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.

Heat Fractures:

Corelle is ideal for microwave use, but in rare circumstances the bottom of the dish can drop off when it is taken from the microwave.  Very often the hole left in the dish is the same shape as the hot food that used to be there, and 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 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 can be heated more slowly on a low 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'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 yellow or orange build-up after years of washing.  Usually this can be removed successfully with the appropriate cleanser*.

Spontaneous brown shiny residue:
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 which were put away in perfectly clean condition can acquire strange brown patches after a period of months or years.  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.

Why it forms is a mystery, and there do not seem to be any clues for predicting whether the brown patches will appear.  But the age of affected pieces seems fairly consistent, and colours from the 1950s and early 1960s are the usual victims.  Often the residue can be washed away, and an appropriate cleanser* might be required to remove all traces.  But just as some colours resist the removal of grey marks, a faint brown stain might remain on some items.

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', and how to tell if it's working:

Many diverse cleansers might work very well, but some products can destroy the colour and lustre of enamels, glass and glass-ceramics.  The manufacturer recommends CorningWare Cleaner & Conditioner.  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 the white unfinished surface of opal Pyrex.  These glass types tolerate abrasives well, and in fact it can take a lot of effort to remove grey marks from them.

Conversely it is most important to be extremely careful with the coloured exterior of Pyrex.  The finish can be fragile, and on certain colours the cleaner may simply remove the shine before removing the stain.  A small test patch is recommended before undertaking a full scale application. 

Fortunately it is quite easy to evaluate whether this product will succeed on a given Pyrex 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 absorbed the stain and it will withstand the cleanser too.  This is true of many newer Pyrex patterns, and some of the earliest Pyrex colours clean up easily too, so the age of the enamel seems less important than the colour.  Removing grey marks and brown stains from shades of red and pink is sometimes impossible.

If there is no progress at all after a few rubs, then the solvent just isn't going to work in this case, and the coloured surface is probably too soft to bear more persistent scrubbing.  At this point, with enough brute force the abrasive component of the cleanser will eliminate any mark imaginable, but certainly this will be achieved by ruining the original shine.  This is a decision best left to individual choice. 

Grey marks on the undecorated areas of Corelle, Corning Ware and Centura can be extremely stubborn, and in these cases the careful use of a stronger abrasive can make their removal easier.  Powdered cleansers like Barkeeper's Friend, Cameo and Bon Ami are good choices for this purpose, but all of these have the potential to leave scratches on the glass or destroy the fragile coloured surface of Pyrex.