Sea horse Roman Mold Blown Bottle;  using a plaster mold

By Marion le Red

Windmasters Hill

This is the process taken to make a roman mold blown bottle like below. Roman mold blown bottles where used for decoration and for function.  My try at recreating this makes a produce that is neither.

The process:

• 1. Make a clay positive

  2. Make a cast negative

  3. Treat and prepare the mold

  4. Try it

  5. Re try it

  6. Re-carve and treat the mold

  7. Re-try

  8. Re-carve the mold

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• 1. Make a clay positive

I started by using clay to make a positive of the seahorse.

After finishing the clay positive I had to go back and remove any places that would under cut, beef up some areas and generally work to make it smoother.

• 1. Make a cast negative

Using plaster a paris I made a negative, casting first one side and then the other half of the positive. The positive was destroyed in this process.

The mold was keyed so when the plaster was poured for the first ½ of the mold there were circular notches put into the side of the mold.  A sprue was also made the side of the blow pipe in the bottom of the mold.  The positive had to dry before making the clay bed to be able to rest the positve on before pouring the 1st half because of the difference between wet and semi dry clay I was able to separate the positive from the clay bed.

When pouring the 2nd hlaf of the mold and split of a different clay wasn placed over the positve and the 1st half of the mold.

• 1. Treat and prepare the mold

After both halves where poured and dried and separated the mold then needed to be cleaned of all loose clay bits.  It also needed to be carved to eliminate any under cuts that may get stuck when the mold is opened with hot glass. Places where the glass would get stuck and deform that blown object.

Once the mold was clean and smooth then it needed to be treated with graphite to keep the glass from sticking to the mold.  Graphite is a lubricant. The mold also had air hole drilled into it using a piece of wire hanger, to allow air to escape from around the piece while it was being blown.

• 1. Try it

The first blow in to a mold is a guess, you need to set up the amount of glass and the bubble so that the bubble is slightly smaller than the mold.  My first try was a 2 gather tube that was flattened.

This try I found that I was not hot enough when I went into the mold so the glass did not expand I did not get and of the fin detail and the tail was very tight blocking air flow.

• 1. Re try it

I did not get any pictures of try 2 , it was too hot the tube started to bend when I was coming out of the glory hole and so I did not get into the mold correctly. But I figured out that the tail was again a blocking point for the air.

• 1. Re-carve and treat the mold

I Re-carved the mold at the tail and fins working on reducing the pinch point at the opening.  Trying to keep the detail of the original but at the same time make the whole mold fatter.

Once the mold was finished being re-carved I sprayed it with spray graphite since it was drier than the first treatment . the first treatmetn used dry graphite and the moister in the mold.

• 1. Re-try

Try 3 and 4, on try 3 There was not enough glass in the head to get the extra detail there but the tail was not so much of a pinch point. I needed to be hotter going into the mold.

Try 4 I made a tube but did not flatten it and then did a slight gather of just the head before going into the mold.  This gave the best detail so far. I also added bit work to add the fin and the snout and spikes on the head. I do not think I got the spikes placed in the correct places. But pit work was part of other fish bottle of the roman era.

• 1. Re-carve the mold … what next!

Looking at the front and back views of the final attempt I can see that the mold is just too skinny. I should have started with a fatter positive.

I did not take into account the thickness of the walls on the glass and that for the patterning to show you need to have thicker walls.

From here I can either start over or re-carve the mold. I am not sure what I am going to do.

I think I will start a new mold but one for a cup and keep it a little simpler before moving on to another bottle mold.

I did not finish the bottle part of the either of these or transfer any of them to a punty as by the time I got to that part I felt like they were not something I wanted to finish.

How does this process compare to the roman process?

The process of above is similar to the process used in roman. They have found examples of molds made out of stone, wood, plaster, and sheet metal as well the more common Terra cotta. The same as today a plaster mold is cheaper to make but is also more fragile. A plaster mold for make grape shaped bottles was excavated at Artashat Armenia in Syria.  (pg45-46 in Roman mold blown glass, Marranne Stern)