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14 Aug

Chilling the Cavity of Cast Iron Bottle Molds

Who, Why, and When

I got a job at the Thatcher Glass Co. in Elmira N.Y. to work in their Central Mold Division as an Applications Engineer and management assistant. After being there for about a year, I was invited to attend a meeting of Glass Plant Managers and Production Engineering people. After the meeting, the V.P. of Manufacturing asked if anyone had any questions. So I asked what I thought was a legitimate question, “Who, Why and When, did they start chilling the cavity of the cast iron bottle molds?” No one could answer the question. It sort of caused a few dead moments, then this V.P. informed me that I had been hired to help with improvements in manufacturing the molds and didn’t need to know the history of things. His reaction made my boss expect that he would be told to fire me. It didn’t happen, and in fact a few weeks later Mr. Burd, came to me and told me that his engineering notes from his former job with Dominion Glass in Canada, which was similar to what he felt my job here should be, were in a trunk over the bottle sample crib and that I was welcome to read and study them, but he didn’t know the answer to my question.

Anyway, I had asked enough questions and put together my own understanding of; “Why” the iron was chilled. I had a friend at the Overmeyer Mold Shop in Muncie, Indiana; that helped me a lot in learning about the castings of mold equipment. Chilling the mold iron was done to create a closer grain in the iron that the glass contacted. This meant a better polish could be obtained in the finished mold cavity.

I also found out that the carbon grain structure of chilled iron was dendritic, and these dendrites were aligned in rows of carbon that ran in a straight line out through the cast iron. This metal form (illustrated below) ran away from that chilled surface for about a ¾” length. This structure of iron has a slower thermal conductivity than regular “A” type graphite in the normal cast iron. This slower conductivity of temperature caused the cavity contact metal to run hotter than the old plain iron molds.

The results were a smoother surface on the glass and a more even blowout of the glass against the mold surface, reducing the effect of the wavy glass that bottle collectors call a Whittled or Hammered look in a bottle. I prefer to call it “Cold Mold Ripple” because it looks to me, like the ripple on water when a pebble is tossed in. This Cold Mold Ripple is usually seen in the lower dip mold section of an old bottle, because that iron did not get hot enough to let the glass blow out flat against the mold cavity surface.

The real truth of the “Who and When” didn’t come to me until a little over 50 years later, when I found the answers in the book: “EARLY AMERICAN GLASS” by Rhea Mansfield Knittle. First published in 1927, two years before I was born.

This lady put together my second favorite book on this Important and First American Industry. The book is a review of Colonial Glassmakers and Glass Houses with a lot of information. Including the, Who, Why and When that I asked for so many years before.

The process of chilling the mold iron cavity metal was developed and patented by Mr. Michael Sweeney. He was involved with several different glass house operations in the area of Wheeling, West Virginia and across the river at Martins Ferry, Ohio.

This is a review of where I found this basic information in the book.

Mrs. Knittle covered some of the problems of mold seam marks and decoration sharpness on decorated glassware being made for the tableware being made in the glass industry. She pointed out that sometimes the glass had to be placed near the furnace hole to reduce the roughness of the mold seam, a troublesome fire polish effort. They placed the mold seams where the decoration would hide the seams, but the sharpness still couldn’t approach the sleek surfaces of cut glass. She pointed out how they tried to cool the molds with metal air tubes between the glass piece cycles and that on occasion their problem was having the glass stick to the mold surface.

In the book on page28, in the first paragraph, she wrote: “The process invented by Michael Sweeney for chilling the inner surface of the mold, explained in another part of this volume, eradicated much of this trouble.”

I had to read on and in chapter XLVII “THE WHEELING-MARTINS FERRY GROUP” is discussed. In the first paragraph of page 394 reads: “Michael and H.R. Sweeney, who later became leading figures in the mid-Western glass industry, established their first house in the northern part of Wheeling in 1835.”

The story continues and at the bottom of page 397: “Just before the Civil War broke out, the house (of Wallace, Giger & Ensell) was on the verge of failure, when Michael Sweeney of Wheeling and James Phillips took over, establishing the well known firm of Sweeney & Co.”

This company grew and added other glass men and developed, including the addition of glass presses, discussed in the Knittle book. In the third paragraph of that page she wrote: “Michael Sweeney has obtained a permanent place in the history of glass-making as the inventor of an improvement whereby the inner surface of the mold is chilled to proper temperature, rendering the metal susceptible of the smoothest polish and giving the product a sharpness of outline almost equal to that of cut ware – a condition never before obtained.”

I had known that this patent must have been obtained around 1862 or 1863. In my quest for and in my studying of bottles of the SARATOGA Mineral Water era, I had decided that this chilling of mold cavities must have developed in the SARATOGA glass production around 1864. It was obvious because of the change in the surface sheen and condition of the bottles. The concept of chilling mold cavity cast iron must have taken off through out the industry.

I have not been able to get a copy of this patent from the U S Patent searches. Apparently some of the patent records were destroyed. If anyone knows of other book reference material on this subject, please advise me.

The following sketch is to illustrate the cross section of a mold and to show the dendritic carbon characteristic and tight iron matrix of the iron next to the bottle mold cavity:
Types of carbon graphite structure in a chilled mold cross-section.

The chilling of cast iron mold cavities was accomplished by placing a cold cast iron form of the objective cavity on top of the drag of the sand casting flask set-up. This iron chill had to be placed so that it lined up with the mold cavity pattern, which was formed in the sand pack of the cope section of the flask set-up. This is best pictured by another sketch in my opinion. After the molten cast iron is poured into the feeder sprue to the cavity in the sand of the cope, the flask assembly has to be knocked down and the old cold cast iron chill piece has to be knocked out of the mold casting half, before it is seized by the shrinking mold casting. This same procedure is done for the other half of the two-piece mold equipment.

Position of the mold in a foundry flask assembly.

The pattern for the chill has to be smaller than the final cavity size of the bottle. The pattern makers have to allow for about 1/8” of the surface metal in the cavity to be machined off when making the final mold cavity. The pattern for the chill has to also be sized to compensate for the shrink of the chill iron casting. There is also about 1/4 th of an inch of iron to be machined off the face of the mold casting.

All of this is simplified considerably from the standpoint of working the casting into the final mold assembly. There are parts to the pattern making that will provide a chill print in the drag for the setting of the iron chill in the green sand of the lower flask. There are special lugs that have to be on the end of the casting to allow for holding the castings while the parting faces are machined. These lugs are also involved when the two mold halves are turned and machined on the outside diameter, (an operation called banding). After the face milling and banding, the mold halves have to be machined on the ends and the initial machining of the cavity will take place in this operation. After that the mold cavity has to be machined to the sized specifications. This is different for all of the different kinds of molds that have ever been made. I put in this basic cycle of mold making because I was involved in the whole sequence of mold making operations. Anyone that is interested and wanting more information, ask me your questions.

Red Matthews

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9 Responses to “Chilling the Cavity of Cast Iron Bottle Molds”

  1. 1

    RED, outstanding job! Great work The page is informative and looks highly professional. I’ll refer your website to a hundred of my bottle buddies.

    Dan (

  2. 2
    Red Matthews Says:

    Hi and a big thanks for the kind words. I am working on another write-up on the subject of Whittle and Hammered used in so many eBay bottle advertisements. It bothers me because that isn’t what it really is. RED M.

  3. 3
    Wil Martindale Says:

    Really great job of solving the cold mold ripple mystery, as I had always wondered why there are so many rippled examples out there, considering that the continuous use of the mold would never allow it a chance to chill.
    It never dawned on me that “chilling” was an intentional process, rather than a random effect due to a cold winter morning’s first few bottles blown.

    It’s especially valuable that you came to this conclusion while working in the industry itself. Doesn’t leave much room for doubt!

  4. 4
    bob Says:

    just stumbled on this very interesting info.48 year mouldmaker for toledo mould and thatcher glass

  5. 5
    mastertonh Says:

    Red, I would like to send you a photo of a pre-1800’s demijohn, would be glad to post a link to photos here but don’t want to violate any blog rules. It is totally cold mold rippled!! Thank you, Heather

  6. 6
    mastertonh Says:

    I have found you also on the antique bottles nework – here’s the discussion thread!
    Thank you, Heather

  7. 7
    Red Matthews Says:

    Hi, Well at this point we have been through a lot of discussion, but I wanted to say that since this bottle was obviously blown in a cast iron dip mold I doubt if there is any cold mold ripple in the glass. I will hope to look when I get my hands on it and advise you then. The wavy thickness of the glass happens when the glass is blown up against the molds inside surface – but the coldness of the metal keeps it from blowing against the molds surface to an even thickness. Thus distorting the light with it’s thick and thin glass in the glass. Thanks for the action this has brought up.
    RED Matthews

  8. 8
    Lisa P Says:

    Hi there, I stumbled on your site while looking up pricing for an “antique” Arrowhead” brand demijohn for my antique shop. Your cold mold ripple is the same type of process/problem that I have in my production of silicone rubber molds and polystyrene resin, that’s how I make reproduction scrimshaw. We have always called that effect: orange peel!!!!For years we have had to preheat our molds and outgas them for a time prior to using them. We just accept that as part of the process. Enjoyed your discussion and will pass this website on to the antique bottle collectors that come to us on Cape Cod. Love to learn about new things. Thanks.

  9. 9
    walt w Says:

    Hi! I came upon this mold chilling and now have a greater understanding. My question though has not been answered. I have lead patterns of bottles and was told these were made in the process of making the mold. I was informed that in the process of making a mold, lead was poured in then remelted to use in the process of making additional molds. I have put together an exhibit of recovered material from the Whitney glass works in Glassboro NJ. There I have located a number of mold warmers made of glass. These make sense. The lead examples I have are from WTco in Millville NJ. Whitall Tatum. Also from that house I have wood examples with the glass house prints. I purchased the items to include in my future exhibits and would like to be accurate as to their use. I have considered that some enterprising individual has the Iron mold and simply cast lead and came up with a story. The patina on the lead looks legitimate, but you never know. Anything anyone could tell me would be greatly appreciated.

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