Molds - Wood, Clay & Metal

Molds used in art and production glass work fall in four major categories: optics, blow molds, press molds, and special tools. In addition there is a special process that is drawn from solid glass casting where a mold is made from sand by mixing it with water glass (sodium silicate) and hardening it with CO2 (carbon dioxide) - examples I have encountered normally involve a large amount of glass at the bottom with a blown form above that base; the mold is destroyed with each object made. 2010-10-12  This page Glassmaking and Glassmakers Page on the history of bottle making has excellent examples of molds and their results and includes a short video of hand blown bottle making 2007-05-26

Optic molds (or optics) are metal open ended tube molds that form ridges in the outside of the glass.  These ridges may be straight lines or diamond patterns, etc.  Most optic molds are solid, but those with complex patterns require a hinged split to remove the glass. Jim Moore makes a blade optic in which metal strips in a frame make the grooves. Homemade cast optic below right and rod optic below left. The primary variations in optic molds are: whether the bottom is open - usually meaning straight sided to the mold - or closed - usually meaning the pattern curves from the sides to the bottom, impressing the end of the glass;  the number and deepness of the grooves in simple patterns; size; and the design of alternate shapes. A straight groove optic can be used to hold and apply straight stringer.  Steinert is a primary purveyor of open optics.

Blow molds are a major factor in production glass and can be used in speeding steps in off-hand glass. Common terms are dip mold, turn mold and pattern mold.
In the dip mold the hot glass bubble is inserted, inflated, and withdrawn, thus being suitable for optics or square or triangular prisms.
In a turn mold the blower (or machiner) rotates the piece in the mold to eliminate mold lines where the halves come together.
A pattern mold, perhaps also described by the number of parts coming together - 2 piece, 3 piece -, has specific shapes inside the mold - a flask bottle with wide and narrow sides with an image of a person or building on the wide side.
Molds may be made of wood, usually fruit wood, soaked in water, or of metal, traditionally cast iron but more recently aluminum. Metal molds are treated inside with a baked on paste, for example resin or linseed oil and cork, that makes the surface more like a burned wood surface.  Blow and press molds must be preheated for the best detail.  This is usually done by simply making a couple of throw away pieces with hot glass so the mold gets up to several hundred degrees.  Glass will stick to a mold that gets too hot. 2004-09-19  The simple mold above is just a steel conduit tube with a welded on bottom, drilled for venting and lined with a cork sheet (and disk at bottom) that is soaked wet before use.  I built it to make small cylinders of art glass to cut open and flatten for painting and fusing compatible with the blown glass.  2005-04-22

Blow molds are used by placing the blower above the mold, often on a box or a couple of steps up, with the pipe hanging straight down and the hot glass sagging below that. Examples may be found here, also. Because some, though not all, molds have restricted tops to make a neck, and because molds may be impressing a design into the glass, molds commonly come apart into several pieces and may hinge off a base or on the side.  Blow molds may be turn molds, for a radially symmetrical shape, which are the topic discussed below, or fixed, often 3 part molds, for impressed designs. Molds may be wood, metal, or graphite.

Press molds are used to make pressed glass and are open faced with a matching plug that drives the blob of glass into the mold, also forming the inside of shape. Pressed glass must taper inside to let the plunger out.  To allow removal of the glass, multipart exterior molds are used, leaving slight seam lines. Information showing machines from the the early history of mechanically pressed glass is here, although a simple machine to apply leverage has been a part of pressed glass from the start as shown here

Special tools may be molds, such as those used to form the necks of bottles. Jim Moore is offering sheet metal fin molds that give a 6 or 8 sided form to the bowl of a goblet when pushed inside the soft glass bowl. 2003-06-24

Jim Moore is now selling metal "blocks" that are thin cast shells with a cork lining.  The material is aluminum per his catalog. [2003-06-24]  They are being shown on Olympic Color Rods site, 1" to 10" $40-$60 2002-09-15

The following is a discussion originally posted on the Craftweb Hot Glass site, edited and copied here with permission of the authors.

Re: cast iron blow molds
[see images just above]

[Previous posting removed on revocation of permission 2009-07-01]

Cast iron blow molds must be lined with a carbon surface.  This can be done with a smoky acetylene flame, but it may be better done with linseed oil that is brushed on, baked to tacky, corked, repeat or commercially is normally done with a special mold paste that holds the cork powder. Visit http://www.renite.com/ and if you need, request the mold paste pdf by e-mail.  The carbonized cork surface is soaked or sprayed with water to provide an addition protective steam surface.  Molds work best when moderately heated, usually by blowing a throwaway piece or two.  [MF general summary from variety of sources 2009-07-01]

Posted By: alex <bigshot@glassattack.com>
Date: Saturday, 16 February 2002, at 3:01 p.m.
In Response To: Re: Cast iron- the source question (Pete VanderLaan- faux moderator)
Those looking for a cast iron blow mold source, try: www.lynchindustry.com/glassware.htm

Alex
Posted By: F.Thumb <dan@flamingthumb.com>
Date: Saturday, 16 February 2002, at 3:03 p.m.
This is where I got my cast iron molds made at (it was a referral from Walter):
West Virginia Mold Shop Paul 304-269-4436

Posted By: F.Thumb <dan@flamingthumb.com>
Date: Saturday, 16 February 2002, at 2:59 p.m.
I don't know if they are still doing this, but several years ago I called Maryland Cork Company (410-398-2955) and asked for some samples of cork dust. They sent me (free) four 1 lb baggies full of cork powder. I'll have to get the pictures of my mechanical boy to post now. Haven't used it yet, but still mean to as soon as I've got our new/improved studio finished. Thanks for all the info on set up.
- Thumb
 

Clay Molds
I have been working on a flip apart mount for a 3 piece mold for some time to make a 3 sided bottle or a log candle holder.  I came across a Penland Ceramics books which gave detailed information on slab building.  Using my old faithful white clay I decided to build some test pieces. 
 I cut outline patterns of the sides and bottom from stiff cardboard illustration board for 6x3 inch forms in 3 ways: no taper, 1/8" taper and 1/4" taper.  These two are the 1/8" and no taper models. [1/8" is off vertical, so the bottom is 1/4" narrower than the top.] The picture shows more distortion than exists in the pieces and the tops were trimmed after taking it.
 The clay was roughly cut from the 25# block with a wire cutter and then rolled with a rolling pin that takes rings for specific thicknesses, 3/8" in this case.  Lacking a special pin, an ordinary cylindrical rolling pin held up with thin slabs of wood with the clay in between them.  The book says he prefers to cut to exact thickness with the wire held on rods with nails at the spacing he uses, because he feels rolled slabs are more likely to warp, but I felt 3/8" was very thin to cut to thickness.  After rolling, the cardboard was used to guide the knife to cut a 45° bevel.
  The slabs were laid on several layers of newspaper to firm up to leather hard, which takes about 10 hours.  When stiff enough to handle, the edges were grooved with a fork and slip was applied with a brush. (Slip is just clay dissolved in water to make a thick cream - it takes a while to get all the lumps out.)  Two sides were lined up and joined and then set on the base.  Then the other sides were added.  If this were a pot, the joint should be reinforced on the inside and the outside worked smooth, but I am making a mold, so a lot more attention was paid to the inside (not shown), getting pinching closed joints, adding slip to smooth, straight lines, etc.  After the clay had firmed up some more, a drill was used to place vent holes (visible in the photo) on all four sides and the bottom.  Now to let it dry and fire it in the annealer and glory hole. 2005-02-09
  After some quavering, I decided to go ahead and make a neck mold for the top, trickier clay work.  Making it separately means I can use these, if they work, as an insert mold and try the neck separately. 2005-02-09

The next step is to make a split mold with an attempt to make something as quick as possible.  When I used the example of a golf trophy in explaining what I was trying to do, I decided to make one.  I decided a bottle 3" on a side would fit my hand nicely.  I cut 6x6" poster board and grooved it to fold 1-1/2" on each side, taping to hold the fold. [bottom row]  On one side, I imbedded a golf ball part way through a cardboard mount.  I added clay slip to be sure the clay got into the dimples on the ball.  I should have oiled or greased the ball as easing it out took some time.  On the other side, I molded the head and lower shaft of a golf iron from Plasticine clay. Here I painted slip in the grooves to be sure they were defined.  In both cases I rolled the grey potters clay to 3/8" thick, cut it to 6"x6" and forced the very soft clay down over the cardboard and molded it around the decorations, working out the bubbles.  The ball wanted to push back through, so I added a lump of clay to support it.  I also rolled a base 4x4" and sliced a curved line through it to align them.
After letting the clay start to dry (approaching leather hard), I worked it off the mounts and carefully aligned the sides to let it harden.  The plastic clay iron came off the mount in the clay. [row 3, right]  Getting the plastic clay out became a challenge because both clays are soft.  I removed most of green plastic clay and then started heating the molds.  I expected the green clay to drip out, but instead it dried and cracked and crumbled when the temp was 210, so was easy to remove.  Doing a web search, the plastic clay used in claymation melts at 150F, but the recipes given for making your own oil clay are wax, grease, and oil with clay or talc (here) and with this much clay, melting won't happen. As I write this, the clay is doing a first heating/firing up to 900F over 3 hours.  2005-04-15  The images in the bottom row on the right show the mold after the firing.  The drilled vent holes are barely visible in the center of the ball and in the heel and toe of the club. 2005-04-18

 I built this press (instead of something involving welding and air cylinders) because of cheapness, materials on hand and incredible frustration that I could not blow glass for a long period.  [These pictures were taken 2005-09-23, look at the dates above and finally blowing glass into them below]  This montage shows the open and closed positions.  The wires on the clay mold insure they are pulled back.  The cutaway on the vertical support clears the protrusion off the ball/club head on the mold. The metal angle at the left end is a step-on to open while the raised section is step-on to close.  The left end wood is screwed down, the right end is held by screws with washers through slots to accommodate different molds.  The only thing I would add is a pair of guides midpoint on the slider (about where the picture seam is) to keep it from slipping sideways. Big, awkward, easy to build, easy to use. 2005-11-11

I blew into these molds and recovered a glass piece (below) that shows all the problems of blowing square and into deep set molds - cracked all over the place and lost one in the glory hole.  The bottom popped off also.  Inclined, in this case, to use a flat plate all the way across the bottom instead of trying to make matching half plates and firmly attach them. The mold of the club head, which was originally made in clay came out with better definition that the golf bowl, which was a real golf ball pushed through a hole in cardboard.  No real dimples at all.  Probably protrudes too far but maybe okay with more practice. 2005-11-11

I have had some clay faces around that I have gradually worked on to improve their looks and I decided to mold this one off and impress it in the clay as above.  This time, I made a wax impression (not shown) to melt out of the back of the clay.  Shown are the latex negative and in investment positive to be used in making a stamp beside the clay shape like the golf club above - the face was put on the same cardboard and again slip was painted into the details before the soft clay was pushed down on the positive. 2005-05-01

The bottle model to the right was partly made with the extruder tool below.
The plaster form was from a nice looking bottle and a half mold for making the two halves.   You can see a sketch of one plan - large star with 2005 vertically. The clay extrusion tool below uses a long 3/8" bolt on hand, several nuts and an aluminum crossbar as a handle. Turning the handle pushes a wooden dowel with a hole to receive the bolt into a short plastic pipe which is glued to a male pipe thread adaptor. A screw on cap with a shaped hole forms the clay being pushed through.  The strain is taken against a ring around the threaded end against the shoulder of the fitting. The ring is silver soldered to bent over ends of steel side rails that have a hole drilled in the U end for the nut on the bolt to push against.  The odd appearance of the side rails beside the pipe has to do with making the rails the wrong length the first time but also allows adjustment between the fittings to the ring and the U portion without requiring precise bending.
  In use, the clay is worked with the hands to soften it, rolled into a rod and placed in the pipe followed by the dowel.  The bolt is adjusted and tightened to push the clay.  For the numbers 2005, the hole in the cap was a triangle made by drilling a hole and then filing out the corners with small files.  20010-02-19

Other Techniques
This site [Roman Glassmakers Articles Page] has explored techniques in Roman Period Glass and says this on a page that discusses other techniques and problems "The most obvious point to note is that the mould has to taper very slightly outwards towards the top (a millimetre will do). If it doesn't, the bottle will not come out of the mould.
If using a sandstone or terracotta mould - as the Roman glass-makers did - it is important to keep it slightly damp. If it is dry the molten glass can adhere to it, particularly as it warms up, resulting in small pieces of sandstone sticking to the bottle. The base is especially prone to this as it is in contact with the part of the parison which is at the highest temperature, and it will result in the loss of the carved detail.
If the mould is too damp, then cooling rings can appear on the walls of the bottle." These people use plain sides and a patterned bottom plate.  They also say that for their reproduction work, they use kiln shelves which they texture like the sandstone, but which last much longer.  2005-11-11