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Cast Iron Molds and Paste Molds

While the student will most often encounter cast aluminum optic molds such as offered by Steinert Industries,
old production glassblowing factories, often used cast iron molds and paste molds to both speed up the production
as well as to produce nearly identical pieces.

Unlike the one-part (one-piece) Steinert aluminum optic molds, the cast iron molds and paste molds are hinged,
which allows for "under-cuts" in the profile of the piece, and assures that the glass bubble on the blowpipe
can be easily removed, after the mold is opened up.

Cast iron molds can range from simple two-part molds, such as used to produce drinking glasses,
to very elaborate three-part and four-part molds.

Four-Part Cast-Iron Heart Mold

My friend David Duthie of Bucks County Hot Glass made the glass below, from the mold above


Four-Part Cast-Iron Hobnail Mold

The handles are 0.75" copper pipe, cut into one-foot lengths, which slip over the threaded knobs.

My friend David Duthie of Bucks County Hot Glass made the hobnail glass below, from the mold above


Four-Part Cast-Iron Double-Bubble Glassblowing Mold

(the mold is wet with KROIL lubricating oil)

(mold closed)
Jeff Vanaman suggested adding handles, which I made from
3/4-10 threaded rods (cut to 8" length) and covered with red electrical tape.
The threaded rods were connected to the mold with 3/4-10 rod coupling.
There is a copper tube (with red electrical tape on the end) which keeps the mold closed while blowing
(it is surprising how much air pressure you can blow through a blowpipe into a heavy mold like this!)


(mold open)

(note the center circle which has an engraved pattern to match the mold)

(detail on one quarter of the mold -- front right section)

It amazes me the attention to detail in the manufacturing of this mold...
Because there are four quarters to this mold, you want to assemble them in the correct sequence,
and there are ONE, TWO, OR THREE nail-punch holes where the quarter sections meet,
to make sure the mold is assembled correctly.
You can see these nail punch holes occur on all three levels (mating surfaces) of the mold!
There is no need to identify the matching of the fourth section,
because that is the section with the pair of handles, extending from the front of the mold.

(photos with and without flash on the camera)

(photos with lights on the inside)
Watch a Movie of the lights flashing inside this glass piece (13.0 MegaBytes)
Red White Blue Globes from Glassblowing Mold
Red White Blue Globes from Glassblowing Mold
Red White Blue Globes from Glassblowing Mold
Red White Blue Globes from Glassblowing Mold
Movie of Red White Blue Globes with flashing Lights (10.0 Megabytes)

Three variations of glasses made from a four-part cast-iron glassblowing mold:

For those who noticed the dolly underneath the mold -- the mold weighs more than 100 pounds (45 kg), and is very difficult to move including because it is hinged and in 5 parts. Karl Carter suggested using a dolly to move the mold around the studio, so I went to Harbor Freight and found their "Mover's Dolly", part# 92486 on sale for $15.99. This dolly, when purchased, is 30" Long x 18" Wide (designed for moving furniture), but I removed the carpeting on the top, unbolted the casters, and used a circular saw to make this dolly the compact size of 18" Wide x 12" Long that you see above, perfect for moving a heavy mold around. I did use one piece of cut-off-wood that was removed, and screwed it underneath, because the dolly, if you put your foot on one edge, had a tendency to tip over because of it's "shorter wheelbase" -- not so much a problem with a heavy mold on top, but why not add that stabilizer just to be safer.


Preservation of Cast-Iron Hobnail Mold

My Friend Jon Burns, who teaches 3D and Sculpture at Bucks County Community College,
recommended protecting and lubricating these molds with Kroil by Kanolabs.

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800-311-3374 contains the following wonderful information:

Turn-molds: The turn-mold is more of a process than a mold and could be produced in about any full height round mold. All turn-mold bottles (also called a "paste mold") are round in cross section since no other shape could be turned or twisted in the mold to produce the seamless body distinctive of these bottles. The inside surface of a turn-mold was coated with a "paste" of organic fiber (often sawdust) which was also wetted between each blowing. Upon contact with the very hot glass the water turned to steam. The steam formed a cushion that the bottle "rode" on while the parison was expanded and rotated by the glassblower. The combination of the steam cushion and the rotation contributed to a distinctive glossy or polished glass surface to a turn-mold produced bottle that other types of bottles do not have, with the exception of fire polished free-blown bottles (Toulouse 1969b; Munsey 1970). However, turn-mold bottles will be very symmetrical throughout; free-blown bottles will not be symmetrical (Jones & Sullivan 1989).

In addition, the granular texture of the paste on the surface of the mold and/or imperfections on the mold surface, in conjunction with the bottle rotation, very often caused the formation of concentric horizontal rings on the body of the bottle. It is known that some turn-molds were made of apple or cherry wood at the Whitney Glass Works (Glassboro, NJ) as late as the early 20th century (Lohmann 1972). Wooden molds would also be more likely to have uneven inside surfaces due to the effects of the extreme heat of the molten glass. Because of the rotation of the bottle in the mold and the wetted paste coating, whittle marks (discussed earlier) are rarely if ever seen on turn-mold bottles (Toulouse 1969b; Munsey 1970).

Because of the rotation of the bottle in the mold (not the opposite as the name "turn-mold" would suggest) embossing on the body of the bottle was impossible; labeling or the occasional blob seal was the only way to notify the product purchaser of what product the bottle contained (Toulouse 1969a) Virtually all turn-mold bottles also have no embossing on the base, though some limited embossing has reportedly been observed. This would have entailed a secondary molding base plate that replaced the original mold base plate after the bottle was rotated in the mold but before it cooled and solidified (Toulouse 1971; Lockhart pers. comm. 2004). In any event, base embossed turn-mold bottles are very uncommon and none have been observed by the author of this website.

It is possible that many or most of the turn-mold bottles sold by American glass makers were imported and not actually produced in the U.S. May Jones, in the first volume of her nine volume bottle history booklets called collectively the The Bottle Trail, quotes a Owens-Illinois Company provided history that notes that their predecessor (Illinois Glass Company) imported the turn-mold bottles they sold and that " the United States, turn mold bottles were not generally produced" (Illinois Glass Company 1903, 1908, 1911; Jones 1961). As noted earlier, it is known that the Whitney Glass Works (Glassboro, NJ) - a large producer of bottles - did manufacture turn-mold bottles with wooden molds as late as the early 20th century (Lohmann 1972). In addition, Toulouse (1969b) notes that patents were granted in the U.S. in the 1870s and 1880s for "seamless bottles." So it appears certain that some turn-mold bottles were produced in the U.S. Although it is likely that many or even most turn-mold bottles were not made in the U.S., they are ubiquitous on U.S. historic sites that date from the 1880s through about World War I, though turn-mold bottles have been documented possibly by as early as the 1850s, but almost certainly by the mid-1860s (Switzer 1974; Beaudet 1981; Gerth 2006). For more information click turn-molds.

There is more information about Cast Iron Blow Molds on Mike Firth's web page here.


At 04:52:11 September 21 2023 displayed this
glassblowing web page at last modified: January 04 2009