Here are mold images, to illustrate the process of inserting the hot furnace blown glass mass in it's interior space, while still blowing air to create the interior shape cavity, once the mold was tightly closed, till the glass hardened and cooled. The remaining attached glass tube was then cut and removed, either from the top or the bottom of the mold, depending on its shape, then the cut was smoothed into a pontil base, or a finished rim. I have chosen a simple bottle form mold to show this craft, old furnace molds, an early 20th. glass plant, and a drawing of how it was done.
Such a complex subject matter, and one not easily covered on an internet site page. I waited to do this till I had a precise idea about the specific information I wanted to discuss. It will pertain to our site's glass fabrication, mostly the hot molded furnace blown type of glass, and the differences with the more recent glass productions, as well as how to study the glass.
After three years of consistently acquiring new glass and doing that, examining the glass and how it was made, it became a very important part of the identification and attribution process for A. Ruckl, as their fabrication is unique. You have to have a large number of their glass pieces in hand, in your own collection, to be able to judge this company's products. Images will not suffice and explanations don't convey enough understanding to most.
Another issue has also become relevant, recent vintage reproductions of Czech era decorative glass, and how to tell the difference between 1930 glass and 1980+ glass. This is pertinent as the Czech Republic and neighbors did re-introduce some glass then, the glass that was popular during the earlier 20th century period. I am including a good glass article by an expert about this, Bill Johns, with permission.
Where Yesterday Meets Tomorrow - Recognizing Old Glass
by Bill Johns
"The study and collecting of glass covers an incredibly vast and complex field. Hundreds of different types of glass were manufactured and are collected. Thousands of different patterns, shapes, and decorations exist. Reproductions have been created for hundreds of years. Millions of pieces copying earlier originals have been produced over that last fifty or so years. They have attained sufficient age to begin to blend in with the authentic originals and have even become collectible in their own right.
Most reproduction glass was not made to cheat glass collectors. It was made to sell to the general public. It copies old glass when those shapes and patterns are attractive to modern buyers. It utilizes old molds because they are much less expensive than new ones. It is mass produced using modern machinery and methods. In these distinctions lie valuable clues to estimating age.
Of course, there are deliberate counterfeits being produced. Effective counterfeiting must duplicate the production methods as well as the appearance of the original. Very rare and valuable pieces of glass were hand made and decorated by highly skilled artisans. The faker must duplicate the hand work and decoration of the original. That level of fakery is expensive and is reserved for Galle, Tiffany, Lalique, and the like. If you don't collect $1000 pieces of glass these are no threat to you. There are also inexpensive knock-offs being mass produced today by unscrupulous "repro houses" that are intended only to be good enough to fool the hasty or ignorant buyer for the two minutes it takes to extract the purchase price from them. It is these pieces that this check list may help you guard against.
So, here is a very non-comprehensive discussion of some of the characteristics to look for in a piece of glass. None of these clues is decisive by itself. You have to take the totality of the evidence and made a judgement.
First, get a good quality pocket magnifyier and keep it with you. A 10x magnifier has about the right power. (It makes the image 10 times larger than it appears to the naked eye.) Typical hand held magnifying glasses are about 2x. They help but don't show enough detail. More powerful lens will show so much detail that you have difficulty picking out the salient ones. Also, be sure to to get one that is "color corrected" so you don't get color distortion around the edges.
Wear. Glass is perhaps the only collectible where you hope to find distinct signs of substantial wear. Look at the areas where the piece touched the table. In use over decades, it was slid back and forth across many different surfaces. That should leave an overall dullness on the contact points, AND numerous small scratches. Using the magnifier, you should see that the scratches are of varying depths, widths, lengths, and directions. That is because they all occured at different times and were caused by different pieces of grit. If you see uniform scratches - similar depth, width, all parallel, etc. you are looking at machine applied marks - either a recent attempt to fake wear or a manufacturer's polishing of the base.
Chips. Over decades of use, minor chipping is almost inevitable - so much so that glass collectors are far more sensible about minor defects than collectors in most fields. With the magnifier, examine the chips. If they occured over a long period of time, they will show variations in the sharpness of the edges and in the brilliance of the surface. A fresh chip will be "shinier" than the unchipped glass around it. An old chip will have taken on the same reflectivity as the rest of the glass. A fresh chip will have sharp edges. An old chip will have gradually lost the sharpness of the break. If all the chips look similarly shiny and sharp, then you are looking at chipping applied to creat the illusion of age.
Crispness. Look for mold marks. Most glass we encounter is shaped in molds. Making the molds is expensive. When a line is discontinued, the molds are not destroyed. When companies go out of business, the molds are often sold to other companies which might, many years later, begin using a mold again. The reissued piece will be the same size and bear the same image as the original.
But, there will be differences. If the pattern was successful one, then it was probably discontinued because the molds got worn out. Injecting globs of 3500ºF glass into a mold many hundreds of times per hour causes considerable wear. Eventually, the pattern becomes weak and blurred. Also, more recent reissues are produced on more modern equipment which operates at much higher speed. Therefore, the glass is in the mold for much less time and does not take on as complete an image as it would if produced at the slower rate of the original issue.
Look for crispness of image and shape. Are the lines clear and distinct? Eventually, lesser lines will wear to the point that they fade away leaving blanks in the image. Are the edges of the piece pointed or showing signs of rounding. And are the characteristics of the edges (i.e. sharp or rounded) uniform over the entire piece? Irregularities indicate worn molds.
Early pieces from the mold will always show crisp image and edges. Later pieces become increasingly blurry. Crispness is an important factor in value, aside from the question of reproductions. Even if a piece is part of the original run, it will be worth less if it was produced near the end of the run because the image has lost its clarity.
Coarseness. A related characteristic is the coarseness of the edges along the mold lines. Early glass was pressed into the mold for much longer and the molds were clamped together with less pressure than is modern glass. Therefore, in old pieces, the glass oozed into the cracks between the molds, becoming raised spots on the finished piece. If you run your finger along the mold marks, you will feel a roughness, even a sharpness. On modern glass, the mold seams are very smooth.
Bubbles occur in all glass but are more common in old glass. In early cut glass, you rarely find bubbles because it was considered a serious defect and the piece was thrown back into the kiln before cutting or the bubbles were cut out during the creation of the pattern. In pattern glass, bubbles are common, since it was glass for the lesser classes. In new machine made glass, the molten glass is mixed more uniformly and injected into the molds more precisely causing far fewer bubbles to occur.
Foreign Matter occurs when dirt gets into the molds between injections. Again it would never be found in cut glass but does occur in pattern glass. Much of the early pattern glass was hand blown into molds. Several apprentices stood around the glass blower with open molds waiting for the hot glass. Dirt got into the molds while they waited and then into the glass when it was blown. In modern machine pressed glass, there is little opportunity for dirt to get into the mix.
Straw Marks are irregularities in the surface which can be seen by looking across the surface from a side angle. They are caused by slow injection of the glass into the mold and are an indication of old glass.
Assymetry of shape is a characteristic of old glass. A goblet might be tipped on its stem, or a bowl might be off-round in an oval or scalloped shape, or a spooner might have a slanted base. Such irregularities are caused by improper cooling. The glass was too hot when it was placed in the cooling ovens an it sagged before completely hardening.
Clarity. Over the years, glass has been made using many different formulas. These formulas cause slight variations in color which, though extremely subtle, can be a strong indicator of age. Most pre civil war glass was "flint glass." Flint is identifiable by its extreme transparency and especially by its sound (more on that next.) It was made using lead as an additive. Once the civil war began, lead was no longer available so soda lime was substituted. This "non-flint" glass is ever so slightly less transparent. As time passed, gradual improvements in the formulas slowly produced clearer glass. The difference is only apparent if an early piece is held up beside a modern piece in the same light. Then you can see that the early piece is slightly "whiter" than the new one. After the war, flint was again produced but never became dominant again because it was substantially more expensive than the soda glass.
Ring. Flint glass is what we sometimes today call "crystal." Tap it with your fingernail and it will ring like a bell. Non-flint will thunk; flint will sing! Of course the shape of the item and where you are holding it will effect the sound, but you can easily tell the difference.
Color. The color in glass is produced by changing the chemical formula used to creat the molten glass. Over the years the formulas have changed as cheaper ways were found to produce similar colors. But the colors are not identical. Careful study and familiarity will allow you to distinguish old colors from new. For instance, modern purple is a much stronger harsher color than the early tinting.
Luster. Glass with color applied to the surface is variously called iridescent glass, opalescent glass, carnival glass, etc. The color is obtained by applying a chemical to the surface and then heating the glass to bond the chemical to the glass and activate its sheen. Recent reproductions have a much weaker shallower color than the originals. Study some authentic pieces and the difference will become apparent.
Feel. New glass has chemicals in it which begin to react with moisture in the air, giving the surface an ultra-smooth or "slippery" feel which is absent in old glass. Again, touching old and new glass will begin to reveal the difference.
Shape. The victorians had glass pieces for every conceivable use (and some that, today, we can't even begin to conceive.) If it's an odd shape it's probably old because there is no modern demand for it. If it is a very common shape - goblets plates, pitchers, for instance - it might be recent because there is a market for it among today's consumers.
And after you've done all this, you still don't have a definitive answer. Maybe it was a gift to great great grandmother on her wedding day and she packed it away in the attic unused for 120 years. Maybe, but not likely. Use your common sense. What does the preponderance of the evidence say. Then make an educated guess.
The only way to learn glass is to study it - touch it, look at it and through it, read about it, and look at as many different pieces as you can. Eventually, you will be confident in your conclusions and glass will become a source of great pleasure.
We have many fine examples of early glass at the Coxsackie Antique Center. (and, no doubt, a few repro pieces skulking about in the dark corners.) Come on in and spend some time looking at the pieces and caressing them and studying them. The only way to really learn glass is to see it and touch it. We've also got an extensive glass section in our Reference Library that you can study. We have some excellent 10x color corrected magnifiers for $12.95. Comparable ones at a jeweler's would cost in the $30 range. And finally, we have a number of excellent glass references available for sale in one of the dealer booths."
(Antiques in Coxsackie #2 March 1997, Bill Johns)
ANTONIN RÜCKL & SONS 1919-1939
Including WILHELM KRALIK & SONS Czech Glass