spectrum - recognizing quality

7/27/2019 Spectrum - Recognizing Quality

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The Clues to QualityCompared to things were used to buying every

day, art glass is an unusual commodity. It comesunpackaged, unlabeled, and in a wide enoughvariety to boggle the mind. Its expensive, too.Telling good glass from bad isnt easyunless youknow what to look for. There are a number of cuesto quality that you can learn to recognize before

you buy. Becoming familiar with them will helpyou avoid the expense and frustration of buyingdefective or poor quality art glass.

The things to look for fall into two main categories:

features that signal structural problems in the glass,and characteristics that will affect the appearance

of your finished product.

Structural Cues

Structural problems in glass translate into poorcutability, high waste and high frustration. Evenif you manage to work around them in cuttingand fabrication, they can endanger the structuralintegrity of your finished project and may interfere

with its appearance. These defects are the resultof one or more problems in the manufacturingprocesspoor composition, incomplete melting,

faulty annealing, and others.

Batch & StonesThe most common (and easiest to spot with closevisual inspection) are stones or batch in the glasssheet. These unwanted aliens are bits of granularraw material that have failed to melt completely.They will appear as tiny grains of sand-like material

on the glass surface (batch) or as small rocksembedded in the sheet (stones). Examine theglass carefully and steer clear of batch and stones!

Where you see a little, there will be more thatare invisible to your eye. Surface batch is a directcue to poor quality glass. It is unattractive andmay interfere with your cutting wheel, but mostimportantly, it foreshadows other less apparentquality problems. Where you see batch on thesurface, there may be stones that you cannot see.

A stone may send your score ripping to oblivion,

and if included in a finished project, could breakthe glass sooner or later.


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Thick-Thin VariationExamine the cut edges of glass sheets before youbuy. Look for irregular thin or thick spots thatare not texture-related. When glass sheets are

formed without careful attention to uniformity oftemperature, thick or thin spots will result. They

represent structural weaknesses that may carryover to the finished project, can contribute to poorcutability, and may cause problems when foiling orleading.

Surface batch appears as tiny grains embedded in the sur-face. Where there is batch, there will be stones.

An internal stone has caused breakage in this sheet.

Visible stones in streaky green cathedral.

Irregular thick or thin spots can be found by examiningsheet edges.


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WarpageOne easy test that will provide a clue to cutability isinspecting for sheet warp. Does the piece of glass

lay flush on a flat surface or can you detect a bendin it? A warp indicates stress in the sheet that youmay discover painfully later on. A slight warp ina large sheet is probably no reason for concern,but if you detect even minute curvature in smallersheets, let your suspicion be aroused.

Mix-FlakingIn multi-colored glasses, each glass in the mixneeds to be compatible with the others to assure

stress-free results. Look at the cut edges of thesheet, paying close attention to the areas wheretwo colors meet. Look for indications that the twocolors are resisting each other rather than mixingcooperatively. It will sometimes appear as though

one color is literally flaking away from the next.

Irregular Breaks & Unsuccessful Cuts

If the sheet youre considering has a broken edge,pay attention to the quality of the break. If itsshape is very uneven and irregular, suspect poorannealing, mix incompatibility, or both.

Also pay attention to the apparent success or

failure of the last person who tried to cut a sheetof glass. Look for scores on the surface that wentunfulfilled, or jagged edges that indicate the

factory, wholesaler, or dealer had problems cuttingthe glass. By examining the score closely, you cantell whether the unsuccessful cut was caused bypoor glass or by poor glass cutting. In the lattercase, the glass was probably over scored (too

This streak of opal is resisting the mix and trying to flakeaway.


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much pressure applied). Look very closely todetect minute chipping along the score line. If itsapparent, over scoring is the more likely culprit

and the glass may not be to blame.

Surface ChillIn the manufacturing process, when uneven

temperatures occur at the point of sheet forming,all sorts of interesting effects result. They canbe attractive to the arty eye and often add anantique-like character to a finished project, butsometimes represent structural weaknesses inthe glass. Before you buy glass with surface-chill

characteristics, know how you intend to use it. If itwill be under unusual tension, or if it will be usedin very large pieces, choose something else andavoid the possibility of problems.

Surface chill takes a number of different forms. Itmay look like healed cracks or clustered waveson the surface.

This piece shows two clues to poor cutability: a very un-even, irregular edge, which is an unnatural way for glass tobreak, and a previously unsuccessful score.

Two examples of surface chill in cathedral glasses. It can beattractive, but be cautious.


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AppearanceBeauty is subjective. The purpose of this section

is not to tell you what is attractive or what isnot, but to help you learn to recognize variouscharacteristics of art glass. When youre equippedwith criteria to judge by, youre in a betterposition to choose the piece of glass best suited to

your needs.

Surface ReflectionJudge glass for surface quality. Examine it inreflected light, tilting it to various angles. Howwould you describe the general surface? Smooth

and shiny? Rough and leathery? Somewhere inbetween? Unintentional textures may be veryslight and subtle, sometimes resembling thesurface of an orange peel or rough leather. Theylimit light reflection and are less jewel-like thanother, more brilliant glasses, which may or may notbe the effect you desire.

Are there persistent surface defects? Look forbatch, pock-marks, accidental air bubbles, chillmarks, and so on. If the peculiarities denote

structural problems, you know what to do. If not,then consider whether they will add to or detract

from the appearance of your finished project, andlet that be your guide.

This surface is filled with pock-marks from a very gaseouscomposition.

Tilted in the light, this sheet reveals a surface like an orangepeel, or rough leather. It may contribute to or detract fromthe appearance of your piece.


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Mix DistributionIn multi-colored glasses, pay attention to thedistribution of the color-mix. Are the colors fairly

evenly apportioned throughout the sheet or arethere large areas of single colors in some parts?You will find some two-color glasses that appearheavily amber at one end, for example, and mostlywhite at the other. Perhaps its the perfect effect

for what you have in mind, but often you end up

with a lot of scrap. Consider the average size ofthe pieces you will be cutting from the sheet. Is itimportant that each one have similar character? Ifso, you will want to be sure the colors are evenlydistributed. Knowing how you intend to use

the glass will help you judge whether its mix isappropriate or defeating to your purpose.

Light TransmissionMake sure to examine glass qualities in bothreflected and transmitted light. As you know,

its characteristics may change considerably indifferent lighting conditions. In multi-coloredglasses, evaluate the definition of colors in reflectedlight. Then do the same in transmitted light. Dothe colors maintain their definition in relation to

each other or do they change in the light? If youknow how youll use the glass, decide whetheror not it will achieve the effects you desire in

predictable lighting.

Some surface textures limit light reflection.

Uneven mix distribution in a sheet of amber and white.


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Insist on QualityThe glass you buy for stained glass crafting

will be quite different from common windowglass. It will be art glass. Most of it is typicallyhandmade, or at least, it encounters manyhands-on processes during manufacture. Thisis part of the beauty and appeal of art glass, but

it is no justification for poor quality. Examineglass closely before you buy. Look for the cues tostructural problems and poor cutability. Evaluatesurface characteristics, mix distribution, and colordefinition. Compare properties in both reflectedand transmitted light. And above all, insist on

quality. Your dollars and the effort you spend on aproject are worth it.

Spectrum Glass2005 Spectrum Glass Co.

P.O. Box 646, Woodinville, WA 98072-0646

An educational guide brought to you by:

Marbles by Bill Hillman, Mansion Glass, Olympia, WA .Photo: Roger Schreiber

spectrum - recognizing quality

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