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View Full Version : Ed's easy explanation of corking a mold


evanhurley
10-29-2015, 04:00 PM
Hey Everyone:

we are having some issues with our pieces being scratched in our steel molds which have been corked using 40/80 cork from Maryland Cork.

Wondering what grit of cork you recommend for a high production mold, and what quantity/style you cork the mold in.

We utilize renite paste, and do three layers of paste and cork, alternating.

Much thanks!

Pete VanderLaan
10-29-2015, 04:32 PM
I would advise you to look in the archives for posts on the subject by Ed Skeels who really knows his stuff on corking. Ed is not likely to answer this post at all having communicated with him last week, I know he's out of pocket on a long term basis . So, go look in the archives using the search function.

If that doesn't make Ed post, nothing will.

Michael Ahlefeldt-Laurvig
10-30-2015, 10:46 AM
My process should also be found in the archives and does not use renite but the cork I use is far finer than 40/80 grit- I don't know the actual grit size but would guess 220 and above,- its like very fine dust. If you blow on a handful, a lot of it will stay airborne a long time
A common cause of scratches even in a good pasted mold is not keeping things clean- like putting the mold on the floor when blowing and then polluting the dipping water with dirt from the floor. Even dirty water in your block bucket. Everything has to be clean and kept clean- change water etc.
Of the other thousand variations, blowing too hard or staying too long in the mold, will do it.

Michael Ahlefeldt-Laurvig
10-30-2015, 10:55 AM
I could have asked if its like in the same place or random? lots of them or few? Consistent every piece or just now and then?

evanhurley
10-30-2015, 01:32 PM
Micheal-

Thank you for your response.
the mold we are having particular issues with has been re-corked three times now, and the majority of the scratches are in the center of the piece.

They are faint, but noticable, and fairly consistant. We blow into it about 1 every 10 minutes. some scratches are deeper than others.

How many layers of this very fine cork do you use? it's less abrasive than a 40/80? What effect do you think begining with 40/80 and then finishing with a layer of 80/200 would be?

Michael Ahlefeldt-Laurvig
10-30-2015, 02:45 PM
Have you found my long description of what I do in the archives?
On a new- graphite- mold it is enough with two layers - it will last several hundred pieces, say on a wineglass cup, on a more complicated mold with sharp edges maybe 50 to a 100- it depends- is it clear glass? Or color on the outside- which is abrasive
I don't have much experience with steel molds, but I imagine its about the same as graphite
i have no experience with 40/80 grit cork- Im in sweden and get the cork from Essemce, and do it like I've said, because thats the way we do it, and it works.

Michael Ahlefeldt-Laurvig
10-30-2015, 02:56 PM
You should recess the air holes with a countersink ( just do it by twisting it with your hands- a drill is overkill) after pasting, it makes ridges around the hole when pasting - the ridges will make marks

Dave Bross
10-30-2015, 05:09 PM
I saved Ed's post on this, here it is:



To Paste a Mold
To Paste A Mold, you will need.


1. Brush
2. Resin
3. Cork or Sawdust
4. Mold
5. Oven
6. Timer
7. Notebook


1. Get some inexpensive, natural bristle brushes. Use one for the day when pasting a mold, then throw it out. They aren't worth the expense of solvent, and you can't really get them clean enough. There will still be resin in the bristles after cleaning, making the brush too stiff to bother with.

2. In North America, you can get resin from Renite (www.renite.com). Their mold past is either LMP, MP, or HMP, for light, regular, and heavy. Get the regular MP. Quart samples have been available in the past, and the small purchase size is one gallon.



The resin is made of "vegetable oil" and who knows what else, with a fair amount of graphite mixed in. The consistency is like heavy corn syrup used in baking. When you brush it on a preheated mold, the resin will warm and thin out so you can brush it out evenly.



After a half full container has been sitting for a while, a skin will form on the surface of the resin. You will want to get plastic coffee spoons or something to cut the skin from around the inside edge of the container and carefully lift it out into the garbage.



Keep the cover on the pail when you are working with the cork or sawdust. If cork gets into the resin, you will end up brushing cork/resin bits onto the mold. When the paste is cooked, there will be a hard bump that doesn't burn back the same as the rest of the cork. This might be a problem on thin stemware when you don't want to see lines in the glass.


If you can't get some prepared mold paste, you can use boiled linseed oil. You will want natural, boiled linseed oil, not synthetic. You will need to boil the linseed again to reduce the oil to a heavier bodied resin. Even if the can says "Double Boiled," you'll want to cook it again.



Use an old saucepan for cooking. An electric single burner hot plate works well. Cook the linseed oil in a well ventilated area, preferably outside away from any structure. To say "boiling" is a little misleading. It will start to smoke before it boils, and the smoke is highly flammable, so you will really want the oil to simmer, not bubble.


I haven't boiled linseed in a long time, so I can't give you an idea of the time. The only way to tell is to let the oil cool (to room temperature), and check the consistency. It should be heavy bodied. You might find that you need to cook it again to get it to the right condition.

3. Cork. Use granular cork. Small mesh granular cork for thinware like wineglasses, and larger size for bigger molds. I only know of one source of cork.



Maryland Cork Co.,

Blue Ball Rd.,

Elkton, MD 21922.

Tel (410) 398-2955 Fax (410) 392-9433.

Their "Granulated Cork, 20/40 Select, 3-4 lbs/Cu. Ft." is good for stemware and small to moderate size tableware. (They also have 20/40 plain, but I've never used it.) For larger ware, "Granulated Cork 14/30, 4-5 lbs/Cu. Ft." will build up the paste thickness faster and hold more water for more mold time. Screen the cork to get the lumps out. Even if it comes from a nice cork shop, screen the cork. Then run a magnet through it.

These are both important steps because, (A.) lumpy cork makes lumpy paste makes unhappy glassblower, and (B.) the grinders shed metal filings which can cause little rings or scratches in the finished ware.


If you just can't get cork, you can slum and use wood sawdust. Cut up some old wood blocks on a table saw and sift it with a 30 mesh sieve for a fine size, or window screen for a more coarse size. When I say wood blocks, I don't mean blocks of wood. I mean the things you shape hot glass with.

4. If the metal mold is bare, sandblast it with a coarse media, and rust the inside with saltwater. You need to get a surface that the paste can hold onto. If you sandblast it, take the hinge pin out and clean the outside of the mold while you're at it. Removing rust and scale now will prevent metal bits from getting in your nice new paste.

For the actual pasting, warm the bare mold to 400 Fahrenheit. While it is still hot, brush on some oil. The heat will cause the oil to thin and run. Brush it out thin and even. It should start to smoke. This is too hot for pasting but that's not what your doing right now. The first thin layer should dry in a few minutes. It won't reduce and dry out all the way, but leave a tacky varnish on the mold. This will help the first layer of paste to adhere.


After the mold has cooled to spit sizzles, brush on another layer. It should be thicker than the first. If the mold is smoking too much, wait. Brush until the resin looks glossy, but no puddles.


Pour the cork onto the mold. Use gloves and tip the mold around until all the resin is covered. Dump out the excess. The cork will start to absorb some resin. Pour more fresh cork into the mold. Not the stuff you just dumped because it will need to be resifted and checked with a magnet. Cleanliness is next to you know who. After the second application and tipping the mold around to cover evenly, pour out the excess.

On the first layer, don't pack the mold with cork and let it sit there, or it might absorb too much resin. If the cork has absorbed all of the resin, there won't be enough to bond the cork to the mold.

5. On cooking the mold. It's much better to cook the mold in a vented gas fired lehr. No fire hazard and the fumes get vented outside. With a tight electric kiln, the elements can cycle on, get glowing too hot and ignite the fumes just as you open the door to peek. If you use an electric, leave the door open just a bit to vent and keep the concentration of smoke/fumes too low to ignite. You will need a good temperature controller, because: Yu kant doo this bi i.

6. About time and temp:
The purpose of cooking is to dry the resin without burning the cork. A lower temp, with longer time, is better.
You want to dry the bonding resin without burning the cork.
You want to dry the bonding resin without burning the cork.
You want to dry the bonding resin without burning the cork.
You want to dry the bonding resin without burning the cork.

Paper starts burning at 450F, so you know that will be too hot. 400f or less is fine. Look at the mold while it is cooking. Smoke is O.K., that's just the oil reducing. The cork may look dark from absorbing the resin. (First layer only) Just so the cork doesn't come out of the oven hard and brittle.


After 90 minutes to 2 hours, remove the mold from the oven. The mold should just be running out of smoke, meaning that the oil has been almost completely reduced.

After the mold cools for a few minutes, you can put on the next layer. You can brush on the resin at a higher temp than you did for the first layer. The cork from the first layer will hold a lot of resin, so hot is nice to help thin the resin. Stand the mold halves on end and let the resin drain. Then cork it with clean cork two or three times as before until you have absorbed as much resin as possible.


Cook the second layer as before.

After the mold is cool, inspect your work. The color of the cork should be about the same as when you started. It can be toasted, but shouldn't be burnt. Lighter color is better. Press the surface with your finger. The paste should be spongy, not hard. If the paste is too hard, it won't burn in as smoothly as you need. If the paste is burnt, you should put a metal mold into a 900 lehr and burn off the paste completely and start over. A graphite mold can be cleaned with an oxy/fuel hand torch. A surface mix torch with excess oxygen works very well. (If you cook a graphite mold in the lehr at 900f it will start to decay)

After the second layer, you will want to burnish (not blow) the mold. Place the mold open face on a bench or the floor and lightly mist the paste with water. Take a pipe or pontil and get a good sized gob of glass, making a cheater gather with a gather over. Let the surface of the gather cool in air for some time and then use the gob to wipe the mold in a longitudinal direction (top to bottom, not in the direction you will be turning when you blow)


Wipe lightly so you don't stick and tear off some of the cork. Re-mist the mold as necessary. This process is very important and might take some time. You want to knock down the surface until it is even and smooth. Use as many gathers (on fresh pipes) as is necessary. If the gob gets too cold, you can push an edge of the glass through the paste and make a decorative groove.

After burnishing the second layer, warm the mold to 350 and paste one more time, same as layer two.

Lightly burnish the third layer, then set up the mold for blowing. It might take 20 blows to work in the paste at first. Don't make product. Just get clear glass gathers and blow balloons into the mold.

The glass gather can be shaped enough to get the balloon prepped, but should still be moving. You want the balloon to move free and easy, like trousers with no drawers*. The glass should fill the mold with a light blowing pressure.

Avoid reheating in the glory hole just before blowing. If you need heat to prepare the balloon, try to work the balloon surface with a damp newspaper before going into the mold, so the balloon surface is cooler than the glass interior. You getting this? Heating just before blowing will present the hot surface of the glass to the paste. Not good. You want the hottest part behind a relatively cooler skin. The idea is to blow a leathery surface against the cork paste to press it down. If the glass is too soft on the surface, it just burns the surface of the paste and the paste forms the glass. You want the glass to form the paste. You take as much time as you need to get straight with this.

Do I need to tell you to wet the mold when blowing? Seriously, some people get an old mold and don't know. Or they get stingy with the water, dabbing it on with a sponge. I saw that once in a video, little art student with her little sponge, dab, dab, dab. Get a bucket, immerse the mold. Or get a pump up garden sprayer, douche, douche, douche.

During the breaking in part, dipping the mold in a bucket of water with dishwashing detergent will help cut the grease and float off any loose bits of cork. And I mean detergent, not lard soap. You can use lard soap during regular blowing, to help lubricate the glass/mold contact. The detergent is to cut the grease.

You should expect to loose a third of the thickness of cork during the breaking in period.

This breaking in allows you to see if the mold needs more cooking. If there is a lot of grease bleeding from the mold, you may need to cook it some more. This is better than trying to break in a crispy paste job, which is really a waste of time. Overcooked paste will never give a good surface.


After blowing the mold a few times, feel the paste again. It should be forming a layer of charcoal, and the paste should be tight on the mold. If you push on the paste and it slides on the mold, push it back in place and cook the mold some more.

If everything goes well, you should be done at this point.


7. Take notes, keep records. Big thick heavy iron molds will take longer than little graphite molds, so be specific in your record keeping.

Simple problem solving:
Crispy black cork, runny resin. Way too hot cooking.
Crispy black cork, hard resin. Still too hot
Spongy tan cork, hard resin. Perfect
Spongy tan cork, grease bleeds during blowing. Depending on how much grease, either O.K. or needs more time in the oven.
Spongy tan cork and you get grease just from touching the cool mold. Back in the oven, needs more time.
Spongy tan cork and it moves when you push your finger into it. Forgot to light the oven.


Blisters in the paste. A series of bubbly blisters in the paste, down the center, about 2-5 mm across is an indicator of too much resin. This usually happens to me on layer two. The surface of the resin has reduced, forming a skin, and the resin under the skin has gassed, lifting a little volcano shape dot of resin with no cork on the mount. It is possible to have a larger blister 12-20mm across, covered with cork. In either case, while the mold is still hot, press down the center of the bubble with a small scrap of paper towel. It may spring back up, but persistence will get the better of it. Usually, there is enough damp or wet resin under the bubble to act as glue if you can just get the thing to lay down.

Did you notice I said paper towel? That will become clear if the towel sticks to the paste for some reason. Rather than try to remove the paper towel and tear the blister open, just leave it. You can burnish or blow glass on a bit of paper towel and burn it away, but a cloth rag might clog things up.

At some time during the first day of blowing, you might want to work on the mold some more. The paste will be forming a layer of charcoal. The charcoal can crumble and small bits will wash across the surface of the mold when you spray between blows. (Assumes a mechanical mold boy with sprayer heads.) If you have crumbs, the glassware will come out of the mold with rings in the glass.

You will need to spend some time at this point. One choice is to burnish the paste with a metal teaspoon or a very large ball bearing. Another is to take a new brush and brush the paste under running water. The dreaded third option, which you will have to do in most cases, it to take 320 grit wet or dry sandpaper (silicon carbide) and sand the mold, top to bottom. Use small strips of sandpaper, not folded over sheets, so you don't make the paste groovy. Of course, this all assumes you want to make a fine product. If you're a gap toothed, booger eating moron, who thinks that because ya'll blown glass for eight years that makes you smart as a doctor, then you're probly blowin into a cut up iron pipe, so what the ****? Hell, everbody knows, no two pieces of handblown glass look the same, so why the hell bother trying, right? Mold marks? Rings? Huh wah? Shit lady, whadda want? It's handblown!


But I digress.


The pasting will take one day. The preparation will take up more time than you like for the second day. After that, the mold should provide good service for upwards of 1000 pieces. Repaste when you feel the mold is getting too tight.


If you have any experiences to share, contact me at the address below, and I will try to include your comments.

Ed Skeels


*Walking to the mold boy, hold the pipe waist high with the head down and in front of you, about knee high. Twist the pipe back and forth to get an idea of the heat-action in the glass. The balloon will flip, flop, back and forth, like elephants' balls. Also called "shagging it out."

Pete VanderLaan
10-30-2015, 05:47 PM
Thank You Dave!. I view Ed's post as a national treasure and I will move it to antiques and classics quite soon. Somewhere in my mind I remember Ed saying "do it this way or you're a turd" ("Although gap toothed booger eating moron" ain't bad). Classic Ed. Always hard to misinterpret.

I have had my ups and downs with Ed and his volatility yet There's a grudging respect there. I still consider him to be the gold standard when it comes to understanding studio tooling and equipment. He's out of pocket right now, on the road reassessing yet again his fragile relationship to a material he truly loves.

This is for me one of two posts I look for again and again over the last plus decade. The other being Dave's post on phosphate opals. There really are some giants wandering out there people and they aren't getting any younger.