Topic: Anybody heard of skeleton molds?

[Patrick]

Hi,
 Today I was in London with an ex Whitefriars Master glassblower. I showed him the image of the jug in question and his immediate reply was " cast on bosses ". He agreed with Adam that any glass going through a aperture in a mould would tend to thin.
 I have a bowl designed by my Grandfather that uses bosses as a form of decoration and you can see clearly the effect they have.
See image below.
    Regards
            Patrick.
Ps. This is hand blown and no moulds are used !

[krsilber]

(I'm surprised and pleased that such an esoteric question is still interesting people!)

Patrick, no molds are used?  How were the bosses shaped?  What are "cast on bosses"?  Were they applied?  Cool bowl!

I think it's great that this forum is full of people who have personal affiliation with the glass industry.

"Gardner's discussion on the use of a skeleton mould has nothing to do with the lenses that you are so interested in."  Sid, Gardner says, "The completed form had a pillared effect, surmounted by a row of convex shapes with a lenslike appearance." Sounds to me like it had something to do with it!

"The discussion may have lost focus on the lens forming" - not true at all!

OK, everybody, can you just keep an open mind for a wee while?  Just try to understand this explanation?  I'm not simply making up something that will fit my imaginary scenario, I'm drawing together the information that I've read about the glassmakers, these pieces, and the type of mold that at least two authors say is used to make this form of glass.  As I understand it, they were uncommon, probably not used much in the last 100 years, and may not have been used much in Britain at all, so I'm not surprised that even glassmakers don't immediately think of them.  I believe it's also important to look at all four pieces I've posted with these bosses to get an idea of the form of the glass, rather than going from a single image.

I must not be describing the technique very well, so I'm going to take another stab at it, providing visual aids.  Please excuse my drawing!  First visual aid (last image below, I bunged up the attachments) is something I mentioned in my last post:  a glass object blown into a metal structure that become part of the finished piece.  Where there are big holes in the metal, the glass inflates a lot, puffing way out, and where there are little ones it hardly sticks out the holes at all.  The more restrictive the space, the less inflation there is. This is based on a piece I saw in A History of Glassforming, and the caption discussed this phenomenon.  Makes sense intuitively, but it's important to understand for the rest of the explanation.

An imaginary visual aid:  you're making a pizza, but before you stretch the dough out fully you put a cookie (biscuit) cutter on a glob of dough and leave it there.  When you stretch the rest of the dough, the glob remains thick in the cookie cutter.  Agreed?

Now for the real stuff (basically a repeat of what I said in my last post).  The second sketch below is a (lame) drawing of part of a skeleton mold.  I've termed the parts that make the ribs, "rib arms" and those that form the bosses, "ring arms."  They are hinged at the bottom, and move inwards and outwards, and the basal ring plate locks them in position.  The ring arms move independently of the rib arms.
 
A slightly inflated parison is put in the mold.  The ring arms contract, "grabbing" a glob of the thick glass.  The parison is inflated with the ring arms unlocked, so they move with the bubble as it expands, and there is little pressure on the glass in the rings to inflate.  It is blown until the ribs are formed by the rib arms, then those are unlocked, the whole mold is opened up, and the item has its basic form.  Then it's shaped and finished as necessary.

Can you all see now how this might work?

Just one more thing.  I'm reposting a detail shot of the bosses on the pitcher, and also another Hawkes vase with bosses that I haven't posted.  Notice how much more regular the optics of the bosses are in the second example, with hardly any distortion, and how the bosses don't stand out from the body of the vase.  The glass is also quite thick.  I believe these bosses were cut, not molded.

[Frank]

Another possible reason no-one has heard of those moulds (yet) is that they are very complicated and difficult to make and maintain with the temperatures and forces involved. Lots of patents exist for impractical ideas, some of which never got past the drawing stage. I like the suggestion of cast-on lenses, it makes a lot of practical sense.

This type of 'How-did-they-do-that' question can take a long time to resolve. People are still arguing about how the Roman Cage-cups were made. But in the process we are all learning a lot too.

[Patrick]

Hi,
The cast on bosses are made in a similar way to a paperweight ball being dropped on to the button. The bosses on the jug could have had none uniform bosses that were later cut and polished as lenses. My friend said that if he was able to check the internal area of the jug's bosses he would know for sure. The inside of a mold blown item usually follows the external shape , therefore there would be an indent. Can you check this  back at Corning ?
 Regards Patrick.

[Sid]

Kristi:

Good work.  This discussion is a good example of how this group brings out good outcomes.  Your initial concept was completely different than what you have found, through research and diligence, to be the tool that was used to make your pitcher.  The glass workers on this board said, correctly, that it wouldn't work as proposed.   You dug further and found that the tool was not static but instead was dynamic which addressed the issue of what would have happened in a constrained mould.  Without the moving parts, the glass blower would have had a mess on his hands.  I could have chosen better words in my posting but stand by them. I have gone back and re-read the Gardner page provided in the initial posting again and still cannot see anything that speaks of the mould making the convex lense shapes.  Now that you have a copy of the entire writeup from Gardner, could you please transcribe it for the group so we can close the loop on this discussion?  Thanks.  By the way, this group has an open mind and loves esoteric - that is why we are still discussing this!

Cathy B.

The skeleton mould that I saw in action in West Virginia was a simple static version with vertical tubes rising in an inverted cone.   It was used as a dip mould with the gather quickly inserted, expanded slightly with a small puff of air provided by the blower, and removed before being blown to the final shape in a foot operated paste mould.  The sole purpose of the skeleton mould was to provide an optic to the glass object.  The blower used a slight turn in the final mould to make the optic a swirl instead of a simple vertical.  From what Kristi has found, they clearly could get much more complicated than that with moving parts. I think it was Masterpiece Crystal that we visited.  They had a goodly number of these in their mould room in different sizes, diameters of tube and number of tubes.

[krsilber]

Wow, thanks Sid!  I was just about to post a reply, part of which was, "Pressed, machine blown, mould blown in a regular mold, cut, cast on...what technique will you guys think of next?  Is the skeleton mold explanation really so hard to believe?"  So it's really gratifying to hear someone say I found the tool!  A believer!

Gardner on skeleton molds:  "The repoussé goblets of German silversmiths were the inspiration for a few unusual pieces made at Steuben in skeleton molds.  The operation of the skeleton mold utilized a principle similar to that of opening and closing the ribs of an umbrella.  The mold was first placed on the floor of the blowing room with the ribs open in a pattern radiating from the mold base.  The bulb-shaped gather of molten glass was placed at the center of the mold, and a sliding ring encircling the ribs was pulled slightly upward, causing the hinged ribs to close like a partially folded umbrella.  This ring held the ribs in place until the glass gather had been expanded into and through the cagelike form of the ribs.  The ring was then lowered, allowing the ribs to fall away from the glass object, which was then finished offhand by adding the applied foot and other elements and fire-polishing the top.  The completed form had a pillared effect, surmounted by a row of convex shapes with a lenslike appearance.  An engraved decoration was usually added to accent and enhance the molded forms."

My explanation was somewhat different, of course.  I don't know if the "ring arms" were also stationary during inflation, but it seems like what I envision would work better.  Maybe Gardner didn't know, or didn't want to try to explain the whole thing.  I'm going to have to get A History of Glassforming and take another look at the mold they show.

(BTW, the very next paragraph is "Pressed Glass" - they did do some!  But they were "relatively few," and they were mostly flower blocks, glass statuettes, and relief decorated panels produced between 1928 and 1932.)

(...and on the facing page are a couple color plates of stunning Steuben diatreta vases!  "People are still arguing about how the Roman Cage-cups were made." (Frank)  Seems to me there's no question that they were cut!)


Cathy B, where in Australia do you live?  I did my post-graduate research on the Atherton Tablelands, lived there on and off for a few years...and loved it!

[KevinH]

I have been following this discussion with an open mind. I can accept that at least some of the items mentioned may well have been made - at least in part - with a 'skeleton mold'.

Unfortunately, although the information provided here, by all contributors, has been good, it neither proves nor disproves, beyond doubt, the use of a 'skeleton mold' for the lens-like sections. Within the quoted text from Gardner, the important section leaves room for doubt (my emphasis in bold):

The ring was then lowered, allowing the ribs to fall away from the glass object, which was then finished offhand by adding the applied foot and other elements ...

That text can be interpreted as meaning that the lens-like parts were also added after the ribbing had been formed and the mold withdrawn from the item.

This discussion is still open until there is definite proof of the means of producing the lens-like parts.

[krsilber]

I agree, the discussion is certainly still open.  I didn't mean to imply that just because Sid or any other person can imagine skeleton molds might have been used means they definitely were.  I was just glad to see someone agree that it's not completely impossible that this pitcher was mold blown, with the bosses not actually confined to the sides of the mold. 

I'd love to hear more discussion!  From discussion like this I learn from you and am inspired to learn elsewhere. 

Kurious KS

[Patrick]

Hi,
 I think that an inspection of the internal area of the lenses will tell us so much. Perhaps a previous comment about the lens being solid is only an optical illusion ?
Regards ,
      Patrick.

[Frank]

(...and on the facing page are a couple color plates of stunning Steuben diatreta vases!  "People are still arguing about how the Roman Cage-cups were made." (Frank)  Seems to me there's no question that they were cut!)

see here: http://www.glassmessages.com/index.php/topic,1015.msg106925.html#msg106925

Those making cage cups now (with the advantages of today's technology) strongly disagree with the newer theory, it would be interesting to read the complete study but the summary linked in that thread does leave it as an open question and Lierke herself says that more research is still needed.