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Mold Tips From My Foundry

Mold Tips From My Foundry

Unless you’re a sculptor interesting in making molds, this post might be a bit on the boring side.  Forwarned, ha, ha!…..

Last week I went to my foundry in Berkeley and brought in my two new molds (small Beautiful Dreamer and Kristie’s Blossom).  I had some questions for the foundry’s mold lady, Gabe, and she is always so helpful.  She knows her job isn’t in jeopardy by me learning the mold trade. She’s been doing mold making for 30 years for the same Berkeley foundry and has a secure job. :-)

On the hand, my other mold lady that works independently wasn’t so open with info sharing. I felt really uncomfortable calling her on one Saturday a long time ago with a simple question about why my first coat of silicone rubber had bubbles developing all over it (figured out later it was water vapor from my water based clay).  She was very awkward with me and hesitant to even answer my question. Part of it was that I was calling her on a non work day, Saturday, (but I needed to!)  It sort of made me not want to use her again for my molds. In the future I may have a lifesize mold that I won’t want to tackle…. but that’s a whole ‘nother subject.  In a way I understand, though, because mold making is her entire living and she doesn’t want to create competition.

Back to the nice little mold tips I received from wonderful Gabe.  Here they are:

Continuous Weave Fiberglass Fabric

Gabe uses continuous weave fiberglass fabric for her mother mold, not broken mesh fabric fiberglass like I use.  She said it might be difficult to buy the continuous weave fabric in small quantities and that I could buy some from the foundry if I wanted.  They buy in bulk huge rolls.  The continuous weave fabric can be split into 3 thin layers.  She showed me how she pulls it apart and sets it on a stack of  pulled apart squares, all ready to make the mother mold.

Reason for pulling apart the fiberglass is to make a lighter mold.  She uses 2 of the layers on the inner area of the mother mold section and 3 layers on the outer edges for strength (foundry men tap mold on concrete sometimes to get the wax to go into crevices of mold so edges need to be strong).

FGR Plaster with No Denatured Alcohol

All this time I’ve been going off a plaster recipe I received from the other mold lady (independent practice) and she had given me that recipe for another purpose, pouring a plaster cast, not a mother mold.   She included using denatured alcohol in the recipe and I just couldn’t recall for sure why but vaguely remembered it was to extend the drying time of the plaster.  Yep, that was the reason as Gabe explained it to me.  NO WONDER my mother mold sections take forever (one hour) to set up!  I work it and work it for a whole hour and it’s like a workout!

Gabe said that she uses warm water and salt, doesn’t matter what kind, to accelerate the speed at which the plaster will set up.  She doesn’t measure the salt, just throws it in like a good cook!  The only time she adds denatured alcohol is if she is doing a large mother mold section and needs to slow down the cure time so that the section doesn’t set up before she gets a chance to lay in all the fiberglass. Plaster will normally set up in about 15 minutes without having the alcohol in it.  Not sure how fast with the warm water and salt.

Brown Paste Wax and Crisco

Before applying the splash coat of plaster for the mother mold, she coats thd partition walls with Johnsen’s Paste Wax (it’s brown in color) and then over the wax on the partition walls and the silicone rubber with a coat of Crisco.  I’ve been using petroleum jelly.  She said that petroleum jelly will be too slick and the plaster will want to run off of it.  Well, that is true but I managed to allow it to thicken enough and then splashed it on good so it would, in fact, stick.  But I may try the Crisco since she likes that.  The paste wax is used on the plaster partition wall to make it easier to pop off any plaster that gets on it from the next  mother mold section.

With a sharp utility knife she razors the edge smooth for the plaster partition wall. (I was wondering how she got such a nice straight edge.)  I think she razors it before she applies the adjoining next section.  That’s how the paste wax comes into play and it seals this cut edge.   When done with the mold she sands the edges.  I’d been doing this too but I’ll see about getting a sharp razor for my utility knife and cutting the edge.

2 Bowls of Plaster

My method up until now has been to make two separate batches of plaster per mother mold section.  Reason being is that I wanted to have the first plaster layer set up some before laying on the fabric with the second batch of plaster.  Well…. Gabe used one batch of plaster put into two bowls.  She pours some of it in a small bowl and then grabs a hand full of dry plaster and mixes it in until it’s a thick goop.  She puts it around all the keys and crevices kind of thickly to fill in those areas and also puts a layer over all the rubber and walls for that mother mold section- it is the first layer.

The second layer is used with the same batch of plaster but without adding extra plaster- this is poured into bowl two, the large bowl.  She dips the fiberglass squares mentioned above into this plaster and sort of wrings them out a bit and lays them down stacking up some of them (not sure how many but I’d guess enough for the mold section and no more or they’d be wasted).  Then she lays these squares into the first thick layer and works them in.  She mentioned the value of massaging in the fiberglass and plaster with your fingertips.  I’d been doing that naturally as it helped get out the air bubbles and gaps underneath the fiberglass, making sure it was pressed in good to first layer.  I think it may also help the plaster to harden and condense the particles more.

Paper Funnel

Gabe uses a paper funnel and fills it with silicone rubber to squeeze into deep undercuts and areas she can’t reach with a brush.

How to make the paper funnel: You can use a smooth brown paper bag or wax paper and cut a big square.  The take one corner of the square and start rolling it until you get a cone shape funnel.  Seal edge all the way along with tape.  Trim straight across with scizzors at the wide end.  Then fill with silicone and fold over the wide end and tape it.  Then, lastly, cut the tip to the orifice size you want.  It’s all ready to squeeze away!  Incidentally, I’d been using a zip lock bag and filling it with silicone and cutting the edge of it to squeeze out in crevice.  A paper funnel I think will give me more control over holding the bag instead of squirming like a fish like the plastic bag with silicone does. :-)

My main mold tutorial is here.  Keep in mind, info from the mold lady in this blog post is the latest info and what I’ll be utilizing in my next mold.

Hope this info will be of help to you and pay forward anything you know about “schtuff” (ha, ha) to others, it feels great!


4 Responses to “Mold Tips From My Foundry”

  1. [...] See some mold tips from my foundry and why I no longer use denatured alcohol but rather warm water and salt to [...]

  2. Roy Leroux says:

    Very good tips.
    I didn’t quite get what you used the utility knife for. Are you cutting rubber or plaster? and where.
    Pictures would be nice. (Are sculptors visual learners?)
    I’m getting into making the waxes from the molds.
    Some foundries take homemade waxes before they take homemade molds.

  3. Tamara says:

    The outer edge of the plaster partition wall is what is razored smooth.

    Yes, I know pics would be nice to understand exactly what I’m talking about and I may take pics at some point but for now, these tips in themselves took a long time to write and my generosity in writing them I hope is appreciated in itself as is.

    My “homemade molds” aren’t viewed as unprofessional molds and are therefore received well by the foundry because they are up to par with the foundries. In the case of Beautiful Dreamer’s, I made the new mold whereas the foundry’s mold failed. My seam lines were so tight the owner of the foundry said he couldn’t even see them. Yes, if it is truly a “homemade mold” and looks like one, a foundry may be hesitant to take it for it is more work for them to try and pull waxes from.

  4. Excellent info, well-presented! Thanks a lot for sharing!

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