Frequently Asked Questions about Molding Glass Insulators

By Mark Lauckner

* What causes underpours?

What we call 'underpours' are hardly ever caused by an insufficient volume of glass in the mold. They are usually caused because the glass cools before it can be squeezed into the small areas of the mold. This is why underpours occur on the base and inner skirt region. This is the area where there is the most amount of mold surface around the smallest amount of hot glass. In old insulators, the plunger depth or skirt length compensated for the different volumes of glass portioned for each pressing. This results in a deeper pin-hole or shorter insulator skirt if there is a smaller portion of glass in the mold, so a true 'underpour' is a very rare occurrence.

* How do objects get inside insulator glass?

Metal objects like nails, coins, wire, and bottle caps have been found in insulators. Metals with a higher melting temperature than the glass will remain intact for a while, but would eventually be dissolved by the hot caustic nature of the glass fluxes. These objects are everyday types of junk, and were probably introduced into the glass tank through shoveling cullet (broken bottles, one with cap still on neck!), floor sweepings, etc..

* What causes creases and lines ("straw marks") on insulators?

Creases and lines are caused when the glass being poured isn't hot enough to melt back into itself in the mold. These are sometimes referred to as 'straw marks', 'straw lines' or 'cold mold' and are lines in the glass surface where cooling has occurred during the pouring and the pouring stream did not completely melt back together during the pressing of the insulator. The liquid glass folds over itself as it fills the mold and the creases are hardened in position when they touch the sides of the mold. The glass does melt back together on the inside, but because the outer glass is contacting the (cooler) mold parts, the glass can't retain the heat needed to completely fuse back together again on the surface.

'Cold mold' is something entirely different. If the mold is not at least 1000 degrees F., then the glass stream entering the mold will cool instantly as it flows over itself. This causes a concentric rippling effect, much like throwing a stone into a pond. They are occasionally seen on the domes of insulators, but are not caused the same way as creases and folds in the glass.

The parallel thin lines which are often seen over the dome and down one side of an insulator are from a long thin 'stringer' which enters the mold before the rest of the pour. This only occurs when multiple insulator molds were being filled from the same ladle, because the glass never actually breaks clean at the end of a previous pouring. It just hardens with a little thread on the end, which becomes the first part to enter the next mold as the pouring begins again. This is evident on lots of HG signals and on lots of Brookfield products too. The thin parallel lines can be followed until they branch away from each other. This is the point where the hardened stringer on the bottom of the pour is attached to the hotter contents of the ladle.

Later, with automatic portioning machines, the glass stream was cut or sheared off for each mold filling. This often left a shear mark about 3/4" long somewhere on the bottom of the mold (on the dome). Lots of the later era glass insulators have this marking on one side of the dome, like Dominions, Whitall Tatums, Armstrong, etc.

* What causes snow in insulator glass?

Snow is small particles of eroding furnace brick which has broken up and flown out in the glass tank. The glass tank bricks were usually a high alumina firebrick, so the main component in the snow would be clay particles. (Modern glass tanks are made from fused zirconia brick). If the particles are larger than a sesame seed, they can cause fractures because the glass contracts around them during cooling. This usually only affects the area close to a surface. 'Pot stones' are simply larger pieces of eroding furnace brick becoming suspended in the glass. Pot stones the size of a dried pea are usually significant enough to always cause a fracture during cooling.

* What causes amber streaks?

Amber streaks are caused by iron oxide (rust) or other ferrous containing metals in the glass. Just one little fleck of rust or scrap iron causes a nice amber streak when poured. If rusty steel or iron ladles were used to pour insulators, rust flecks would be left in the glass tank after each time the ladle was dipped in. These flecks would melt and make little regions of dark green glass waiting for the ladle to come back and scoop them up for the next pour. Iron oxide is one of the oxides used for coloring glass green. When we refer to amber streaking in glass, it's usually this iron oxide green that we see.

* What causes milk streaks or "Jade Milk" coloring?

'Milk' or non-transparent streaks are the result of accidental contamination by a chemical which acts as an opacifier in the glass, or the deliberate addition of these chemicals in the previous furnace batch to produce opalescent or opaque glass (see two-tones). Small milk streaks in an insulator would certainly suggest undesired contamination in the glass tank, while the fully swirled milky insulators (like the Hemingray-9 and 12's) would suggest that transparent glass was loaded on top of a previous run of opaque or 'milk glass', and that some mixing action in the glass tank had partially blended the two together. The chemicals used as opacifiers in glass were usually calcium fluoride (fluorspar), fluorine, calcium phosphate, or sodium phosphate. Nowadays, to obtain the right balance of phosphorous oxides in the glass, Tri-Sodium Phosphate (TSP) is the main opacifier used. The fluorides and fluorines are used to produce the opalescent glasses which have a orange-ish tint when held up to a light source (will pass light like the opalescent Hemingray and Fry insulators). A little TSP in aqua glass would result in the 'Jade Milk' Hemingray products.

* What causes bubbles in glass?

Tiny 'seed' bubbles are the result of incomplete glass ingredient melting. The alkali which dissolves the silica in the melting action creates small gas bubbles. These bubbles take a long time to rise to the surface and pop, and usually become suspended in the molten glass. They can be squeezed out by reducing the temperature in the furnace and then raising it again. This is referred to as 'pinching' the glass, but wasn't done very often with old insulator glass because optical clarity wasn't required on pole tops. For any bubble bigger than a seed bubble to be trapped in glass, the air must have been trapped very close to the time the glass was being poured and cooled. Most bubbles are trapped during pouring and are pressed into place as the glass cools in the mold. Large bubbles rise and pop on the surface, so they simply don't exist in the glass tank.

* What causes vapor bubbles in glass?

When little flecks of metals (with lower melting temperatures than glass) get into hot glass, they melt and emit a vapor, causing a little bubble around the fleck. Various metals melt within the temperature range of liquid glass (2400 F). When flecks of metals get imbedded in hot glass, the vapor pressure of the evaporating metal causes a bubble. When the glass cools, the metal vapor condenses on the inside of the bubble cavity. The metals condense back to their original colors, for example, aluminum looks like shiny silver and copper looks dull reddish-brown.

* What causes black bubbles in glass?

Bubbles with black insides are carbon or graphite. They would be from sawdust or small wood chips getting trapped in the hot glass. As they burned, a gas was released causing the bubble. When the glass cooled, the vapor condensed and deposited graphite (carbon) on the inside of the bubble.

* What are those little drip points on some inner skirts?

These are air vents. Little holes are drilled into mold parts which have the potential to trap air during the pressing. The inner skirt is often the only place on the pressing where there is a surface with no moving mold part. The small spaces between moving mold parts allow trapped air to escape during the squeezing of the glass in the mold. Several styles of Brookfield insulators were pressed in this type of mold, and no mold line can be found on the inner edge of these inner skirts.

The Coloring of Glass

* What makes Carnival glass?

Carnival glass is actually clear glass with a spray coating applied while the glass is still very hot and fresh out of the mold. This spray consists of metal salts and is applied as a liquid. The liquid evaporates upon contact with the hot glass, leaving the remaining metals attached to the glass surface.

Folks in the 70's in the insulator hobby have been coating clear insulators with a metal evaporation spray but it has more of a silvery look than the real 1940's carnival coating. The new components of the spray are usually stannous chloride, or tin oxide. The original orange-colored spray is made of different chemical ingredients. Because of the forgeries, for obvious authentication reasons, I won't describe the original orange carnival ingredients.

* What makes color in glass insulators?

Metals, mostly metal oxides, produce color in glass.

Unintentional coloring:

This is the result of melting raw minerals contaminated with trace metal ores. It can also be the result of contamination from the melting equipment. A good example of this is the pioneer blackglass. This glass is actually very dark green and is the result of heavy iron contamination. Melting glass in an iron furnace pot produces this over-saturated 'blackglass'. The more common accidental coloring is the result of excavating silica sand deposits contaminated with iron and copper oxide impurities. Mined silica sand for glass making needs to be extensively washed and cleaned in order to produce good quality glass. This simply wasn't required for insulator production. Varying degrees of iron and copper oxides produce the vast array of aquas and greens we see in insulators.

Intentional coloring:

Perhaps the most common intentional coloring of pioneer insulators would be cobalt blue and amber. These insulators are usually from glass companies using bottle glass to press insulators. Cobalt oxide is added in very small amounts to glass ingredients to produce the cobalt blue color. It produces cornflower blue in very very tiny amounts. Amber is produced with sulfur and iron oxide. Amber and cobalt bottles were intentionally made for medicines and liquids which required some UV protection.

Some blue and amber insulators were marketed in the early 1900's as line markers and would have been intentionally colored. These H.G.Co. and Hemingray insulators wound up being various shades of yellow, amber, red amber, brown amber and also electric blue, peacock blue, and various cobalt blues. Yellow can be produced by adding sulfur to the glass batch. Very small amounts still produce the yellow color. It's an inexpensive additive and is widely used. Larger amounts mixed with small amounts of iron oxide produce the golden, red, and deep rootbeer ambers, while the electric blue is achieved with copper carbonate. Turquoise blue is the result of mixing small amounts of cobalt oxide with the copper carbonate. 'Hemingray' blue requires slightly more cobalt in the copper carbonate. Iron oxide (better known as rust to all of us who own older cars) by itself produces emerald-olive green.

Later era dark amber and white glass insulators, like Armstrong and Whitall Tatum, a few Hemingrays and Maydwell 20s, were intentionally colored in an effort to compete in the porcelain insulator market.

Sun coloring:

Glass insulators can be colored various shades of purple and straw by prolonged exposure to the sun. This purple color is often referred to as SCA (Sun Colored Amethyst) but since it is generally not known if a specific insulator is "SCA" or just light purple, it is better not to use the description "SCA" but to describe the intensity of the color (purple tint, light purple, purple, etc.).

Purple colored insulators:

Manganese oxide when added to glass basically makes light gray in small amounts. The gray was encouraged because it simply masked the green-aqua tint produced by small iron oxide impurities. More than 200 grams/100 pounds starts to produce a dull grayish-burgundy color. There is a saturation point at about 300-350 grams/100 pounds which produces a strong burgundy wine color. This color is only available in the molten glass for a while, then it 'cooks down' and fades to back to gray in a rich flame gas furnace environment. This is the kind of glass used to press insulators that have turned purple in the sun. That is, glass containing various amounts of manganese oxide which has been 'cooked down' (reduced to gray), then pressed and allowed to cool. After years of service, the purple color re-emerges.

Straw colored insulators:

The selenium and cerium oxide additions to glass were more recent attempts than manganese oxide at hiding the aqua tint produced by iron impurities. It is assumed that the selenium and cerium oxides produced clear glass, and the exposure to the sun resulted in the straw coloration. Rumor has it that WW1 saw an interruption in the supply of manganese to North America because it was being mined in Europe. This I cannot confirm, but dated glass from after that period seems to be more straw than SCA, and glass from just prior to that period (1914-18) seems to include the purples. Perhaps certain glass factories had accumulated a lengthy supply before the war.

Jade Milk colored insulators:

See the section What causes milk streaks or "Jade Milk" coloring

* What causes two-tone colors in insulators?

Two-tone insulators are the result of one color of glass being loaded into an almost empty furnace on top of another color. When the area lower in the tank is scooped out to pour insulators, the region where the two colors meet can be the gathering area for the ladle. The pour then enables both colors to enter the mold at the same time.

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Last updated Sunday, May 16, 1999