In Part I of this series, I gave a (long-winded) description of designing, making and refining a model for a pate de verre garden panel. It’s about 5×7 inches and maybe a half-inch thick (or a bit less), meant to hang on the wall. In Part II (this post), I’ll talk about making the refractory mold. Part III will cover filling the mold and firing. Part IV will discuss demolding, cleaning and finishing the casting, and display options.
In the last post, we wound up with finished models, ready to be used for mold-making. Now we needed to make the molds. To do that we’ll:
- Prep the model for casting
- Make a mold box
- Mix up refractory plaster and pour the mold
- Remove the model and clean up the mold
Equipment and supplies needed
We got off easy with the first step, model-making, because most of the stuff we needed was already lying around the studio. Mold-making gets a bit more complicated.
Tip: Assemble ALL the equipment you need for each step, and have your supplies on hand BEFORE you start. Otherwise, your mold could be ruined while you’re hunting around for gloves…
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Probably not a great surprise that the tools that you used to make the model also come in handy when you’re prepping it. You’ll be smoothing things, making last minute changes, etc. These include:
- Wire/ribbon clay cutting tools
Now for the new stuff:
- Scissors and a razor blade, Xacto knife or utility knife for trimming mold boxes, opening plaster tubs, etc.
- Roofing tarpaper. Yep, you read that right. Go to the hardware store and pick up a 30- or 36-inch roll of roofing tarpaper for about $20. It’s black, heavy, and it makes excellent mold boxes to contain plaster. Other casters simply use glass panes and/or clay walls to contain mold mix; if you do that you’ve got to seal the base to prevent leaks. It can also get perfectly good clay you could be sculpting all gunked up with plaster. Instead, we’re going to be constructing a five-sided box to hold our model and fill up with mold mix. A single roll lasts me 4-5 years, so you might want to go in with a few friends if you’re not planning on doing a lot of open-faced box molds.
- Yellow or white china marker for marking plaster levels and such. It needs to show up on black and write on rough surfaces, i.e., the tarpaper.
- Mold walls. The roofing tarpaper is too flexible to hold plaster by itself; have special acrylic retaining walls that I clamp together for box molds. You can do the same thing with boards, walls, boxes, cinderblock, whatever you have.
- Old paintbrush. Get the cheapest soft-brushed paintbrush you can find. You’ll be using it to brush plaster onto the model and eliminate bubbles.
- Spray lacquer. This isn’t strictly necessary, but it seals the clay and plant parts, and makes them a bit easier to remove from the mold. Any cheap, fast-drying lacquer from the hardware store will work. I usually get clear, but there’s nothing that says you can’t use red or puce.
- Stapler. To staple your mold box together. If you can get one of those long-throated staplers, so much the better, but any office stapler will work.
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Remember what I said about having all this stuff on hand BEFORE you start mold-making? It’s the most critical for this process; I’ve ruined molds and had to start over because an important tool wasn’t where I needed it.
One way to make this go faster: Make your workspace as ergonomic as possible. If you’re making one small panel, it’s probably not critical, but if you’re making more than one mold, or you have back/neck/knee/wherever problems, do NOT set everything on the floor. You will be staying bent over for extended periods, and lifting heavyish things–not good for your back.
Do a little dry run with the mold mix, water, mold box, etc.: You want to be able to access everything without bending over (or bending over as little as possible), and without getting into stretchy-unbalanced positions. So…get a few extra buckets or low tables, and use them to raise everything off the floor. I have been known to keep supplies on top of my Skutt GM-1414 (although that’s not a great idea); with its stand it’s just exactly the right height for reaching over and grabbing something.
If you can, put your mold box on a turntable or something that can spin–you can be pouring mold mix and rotating the box for a far more even pour.
Bottom line: The more comfortable your workspace, the better your mold will turn out. (Not kidding–when I’m relaxed and not achy and anxious, I make FAR better molds)
This is the stuff you’ll need for moldmaking:
- Mold mix (see the next section)
- Plaster bucket(s). Get at least two 3-4 quart plastic buckets for plaster. You can find these at hardware stores, paint stores, and sometimes just by recycling from something else. I prefer the kind with a wire handle–they allow me to quickly settle the plaster/water so the plaster slakes evenly. Don’t get paper buckets–they’ll break down before you pour. I have two ready in case the plaster sets up in the bucket while I’m in the middle of making a mold. Two also allows you to make more plaster–start one bucket, and while it’s slaking, start the second. You can pour the first, and the second will be ready for mixing and pouring when you’re done.
- Plaster scoop. I have an old candy scoop now that’s perfectly sized and easy to clean, but I’ve used empty tuna cans, old mugs, etc.,–you just need something that will allow you to scoop plaster.
- Sifter. A cheap wire sifter (because you shouldn’t expect to take this back to the kitchen) is best. You’ll be using it to evenly sift plaster into the water.
- Disposable gloves. Refractory plaster is very drying to skin. Many casters mix and apply plaster with bare hands; one or two castings and my fingers start cracking and bleeding, so I’m not one of them. Besides, it’s easy to clean up plastery hands when you’re wearing gloves: Pull them off, inside out, and you’ll contain the plaster inside the dirty glove. You’ll appreciate that when the plaster is setting and you need clean hands, fast.I buy nitrile gloves in 200-pair boxes for around 10 cents per pair. I’ll usually go through a couple of pairs for a simple box cast; when I’m prepping for a show I blaze through these at an alarming rate. If you have trouble putting them on, look for the surgical variety (they’re more expensive)–the insides are coated with talcum powder or something to help them slide on.
- Cleaning buckets. As in model-making, these buckets allow you to clean your tools without stopping up the plumbing. Fill two 5-10 gallon buckets halfway with water. Use the first bucket as the “dirty” bucket–your plastery stuff will go in there to be cleaned. The second “clean” bucket is for rinsing off the cleaned tools. When the dirty bucket is too dirty, set it out in the sun, make the second bucket your new “dirty” bucket and start a third bucket. Let the water evaporate out of your dirty buckets–when it’s gone, the remaining dry cake of gunk can be thrown out. The number of cleaning buckets you need will depend on how many molds you’re making.
- Stiff cleaning brush to scrub buckets and tools. You won’t be using it for anything else, ever again.
Scales. Don’t expect to use your kitchen scales and take them back to the kitchen when you’re done (unless you can protect them with plastic or something). It’s best to buy a set of studio scales–you can pick them up on eBay or the thrift shop, usually. You want it to be capable of measuring 1-10,000 grams. It’s also a good idea to get scales that don’t automatically go on standby after 30 seconds, especially if they zero out the weight when you wake them back up.
- Trash can. I have a marvelous wheeled trashbin that I can line with a lawn bag and move around the studio; if you can find one of those, wonderful! In any case, you’ll want a trash receptacle that you can move close to your work area and line with a drawstring trash bag. The faster you can get plaster crumbs, clay and such into the bag, the less chance of contaminating something in your studio.
- Power drill and jiffy mixer. Many casters say this is optional, but there’s a fair amount of industry research that says not; a few minutes of power mixing makes a stronger, more powerful mold that’s less likely to cause problems. I bought a cheap drill that is ONLY used for refractory mixing, and a jiffy mixer from the hardware store. There are other (more expensive) mixing tools you can use with your drill–they’re bigger, with more surface on the rotors–but my little one works just fine.
- Extension cord. It’s a good idea to have a heavy-duty extension cord in the studio anyway. Having one handy lets you move the drill around as you’re mixing.
- P100 respirator and clean cartridges. If you’re only doing this once or twice, the hardware-store face masks are fine. So many things in glassmaking shouldn’t be inhaled, though, that it’s a good idea to invest in a comfortable respirator. Change the cartridges whenever it gets a bit hard to breathe in the mask. Greg Rawls has a great discussion on this on his safety site.
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This is a simple, smallish box mold with no special requirements so (almost) mold mix will work. A 50-50 No 1 plaster/silica flour mix will do just fine, or any reasonably priced commercial refractory mold mix (not a “permanent” mold mix). We used my own mix, 50% alumina hydrate, 45% hydrocal or dental stone and 5% EPK (a kaolin). I like it because I get a better finish and stronger mold, with a smoother release, but we mostly used it because it’s what I had on hand. It’s not necessary for this project.
You can buy 50/50 plaster/silica from many glass companies–and I do, in a pinch–but it gets kinda pricey. Plaster and silica are commonly available in bulk at ceramics supply houses, and if you mix them yourself they cost a fraction of the price of commercial mixes.
Caution: Refractory plaster are hazardous to breathe. If you mix your own, do it outdoors, if possible, and wear your P100 mask and gloves (mold mix is very drying).
My friend Les Rowe-Israelson has a neat trick where she creates negative pressure by attaching a shop vac hose to a box, turning it on and then mixing up her refractory inside the box. It works–I still like having it mixed somewhere else.
Pate de verre techniques tend to scrub wet, abrasive frit particles against wet plaster, which can dissolve little bits of plaster water into your glass and give you a scummy-ish surface. I’ve also found that if I use commercial mold mixes that contain a lot of grog or sand, such as Ransom & Randolph 910 (otherwise a fabulous, very strong mix), the scrubbing action wears off enough of the smooth surface that the sandy stuff breaks through and can become embedded in the glass. So…save those for another project.
Plaster of paris, or hobby store plasters really aren’t appropriate. They tend to shrink too much as they dry, and can leave the surface looking like a dry lake bed.
Mold strengtheners. People frequently add grog, fiberglass or other strengthening additives to plaster mixes (and I do when I’m making handbuilt molds, or bigger box molds). The reinforcement prevents walls from cracking from the weight of the glass (or disasters). Here, though, we’re not using much glass, so it’s not necessary.
Kilnwash. I add a handful of good-quality kilnwash to the mold mix as I’m sifting in the plaster. It gives me a slightly better release of the glass, and seems to reduce the roughness of the surface.
If I’m really concerned about the surface, instead of mixing kilnwash into the refractory, I’ll make a 2/3-strength kilnwash mix, pour it into the cleaned mold, let it sit for a count of 10 and pour it back out. That’ll coat the mold surface just enough to give a satin finish, without destroying detail.
However, it’s probably not necessary for this project and, in any case, I’d wait to try it until I’d made a few panels. If you choose to do that, do NOT pack and tamp frit against the mold surface; you’ll probably scrape up kilnwash that will embed in your glass.
Custom-mixed refractories. If you really get into glass casting, some ceramic supply houses will mix and bag refractories to your specification. In the northwestern US, Seattle Pottery Supply mix, package and ship my favorite mold mix for a very reasonable price.
I order a few hundred pounds at a time. For a couple bucks extra, they’ll package it in EXTREMELY convenient airtight, 20-poundplastic containers (instead of 50-lb paper bags). The containers are far easier to move around, and stay sealed until you need them.
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- Ribbon tools for removing clay and trimming the plaster.
- Small carving tool (optional). If you want to carve additional detail into your mold, you’ll need some kind of tool to do that. It can be a small clay wire tool, an awl, a nail, etc.
- Stuff to pick out detail with: Toothpicks, bamboo skewers, an awl, nails, etc.
- Tweezers. If you have a pair of surgical or sharp-pointed tweezers, great. They’re wonderful for pulling out plant parts.
- Water sprayer. All you need here is a cheap spray attachment on a garden hose–you’ll be using it to clean out your mold. A waterpik also works but do NOT use a pressure washer–it’ll blow your plaster mold apart. (Ask me how I know this)
- Good light. If you have a strong flashlight or a good desk lamp, great. It’s useful for shining into the mold at different angles to see if you’ve missed anything.
Prepping the mold
1. Make your mold box
These pieces have a 5 x 7 inch (or slightly larger) footprint and should max out at about 3.5 inches high or so. That means you’ll need a tar paper sheet that’s about 16 x 20 inches or a bit bigger. Unroll about that much, and use your utility knife/razor to cut it off the roll.
Lay it down on your work surface and set your model in the center. Go around the base of the model and push the soft clay down onto the tar paper, sealing the model to it. This ensures that plaster won’t get underneath the model.
Use the china marker and ruler to draw a line about 3/4″ to 1″ away from the model. This fold line will become the bottom edge of your mold box.
Now take your scissors, and with the back of the blades, firmly score along the fold lines. DO NOT cut or tear the tarpaper. Then place the ruler at one corner of the tar paper and the corner made by the lines you’ve drawn, and draw another line, connecting them.
Cut along that line to just in front of the fold line–DO NOT cut past that point. If you do, your mold box may leak plaster and make a mess.
Now, gently fold up the sides of your box along that fold score you made. Roofing tar paper is stiff, so the score makes it much easier to fold and shape the box. Pull up each side and fold it inward, creasing it sharply so that you have a 90-degree angle. (be careful not to poke your clay with the corners of the tar paper; they’ll make a mark)
One side at at time, staple the ends of the tar paper together so that they form a five-sided box, open at the top. There should be a 3/4 inch to 1 inch gap between the model and the box walls. The walls will probably be uneven; that’s OK. Take your scissors and trim them until they’re even–there should be at least two inches between the highest part of your model (with the plant parts) and the top of the box.
Check the bottom corners of the box; if you can see daylight, you’ve got a potential spot for mold mix leaks. You can seal the holes by pressing a blob of clay into the hole and smearing it across the tar paper.
Prep your mold box for pouring by setting it in a snug-fitting container (like an old cardboard box), or strengthening the sizes with firebrick, cinderblocks, or even by sticking it up against the wall. (I sometimes use berry box flats). If you pour plaster directly into the tar paper box, it will flex and spread, and you’ll wind up using a lot more plaster than you need.
When it’s set up, set your ruler on the highest point in the model. Measure an inch higher than that, and transfer that measurement to the side of your mold box with the china marker. That mark will tell you how high to make your plaster mold.
2. Seal your model
Take your model/mold box outside, or put it in a large cardboard box (or, preferably, both). Hold the spray lacquer at least a foot from the model and spray it lightly, making sure you coat as much of the model as you can reach. Do the sides and top, including the plant parts.
It’s not strictly necessary for these projects, but it smooths out the surface of your model and seals fuzzy parts or tiny crevices in the plant material so they’re easier to remove from the mold. If you use colored lacquer, the entire composition becomes monochromatic, which can help you in “seeing” the textures and details in the piece. That comes in handy for choosing glass colors.
You’ll only need a single coat; keep it as even as possible. If you overspray and get drips, use your paintbrush to quickly smooth them out. It should dry within a few minutes.
Pour the mold
1. Calculate how much refractory you’ll need
If you’ve made too little plaster to fill the mold you can make more (we’ll talk about that in a bit). If you’ve made too much, however, you’ll either succumb to temptation and pour it ALL in the mold (thereby creating a very thick base that insulates more than the sides), or you’ll simply waste plaster.
It’s not a huge deal with these panels, because they’re small, relatively thin, and symmetrical–if you have twice as much mix as you need you’re not out much. Still, it’s a good idea to get into the habit of calculating ingredients before you start mixing them.
Disclaimer: A lot of variables can affect the consistency and volume of refractory mold mixes, including the components of the mix, what else you’re adding, humidity, the age of the mix, temperature, etc. I’m merely giving a ballpark idea of amounts. As you gain experience in moldmaking, you’ll get a better idea of how this relates to your particular work.
If you’re buying a commercial mold mix the manufacturer usually recommends a water:plaster ratio. If it doesn’t, or if you’re using your own mix, start with a ratio of 60 parts water to 100 parts plaster, by weight. If you had a cubic foot of mixed refractory at that density, you would therefore need about 68 pounds of plaster and 41 pounds of water. (BTW, one of the companies that makes No. 1 pottery plaster, USG, has a nice data sheet on water:plaster ratios and such)
So, knowing that, if you know how many cubic feet of mold you need, you can multiply that volume by those numbers, and obtain the weight of water and mold mix necessary.
- Measure your mold box (length, width, height up to the mark you made on the tar paper wall)
- Multiply those numbers to obtain the total volume of the mold box. So if those measurements are 7,” 9,” and 5:”
7 x 9 x 5 = 315 cubic inches
- Now take the volume of your model (just the clay block–don’t worry about the plant parts, this is ballpark):
5.5 x 7.5 x 3 = 124 cubic inches
- Subtract the model volume from the mold box volume to obtain the volume of the mold:
392 – 144 = 191 cubic inches of mold volume
- Convert the mold volume to cubic feet:
191 cu inches / 1728 cu in per foot = .11 cubic feet
- Multiply your mold volume by the weight of plaster and water in 1 cubic foot:
68 lbs * .11 cubic feet = 7.5 lbs of plaster for this mold
41 lbs * .11 cubic feet = 4.5 lbs of water for this mold
If you’re not into math and you’ve had some practice estimating mold volumes, there’s a simpler way:
- Put your clean, empty plaster bucket on your scales and tare (zero it out)
- Put in whatever you think is the right amount of water to make this mold
- Note the weight of the water
- Weigh out a bit less than twice that amount of plaster
You’ll get roughly that same 60 water:plaster ratio. The problem with this method is it doesn’t tell you how much mold mix you’ll actually need.
This is only a starting point; you’ll want to try making a mold or two, taking them through the firing process and seeing how they perform. Based on that, you can adjust your mix and processes. (Which is another whole blog post)
2. Mix your refractory
Now that you know about how much mold mix you’ll need, set up to make the mold. Assemble all the materials, plug in your scales and the drill/jiffy mixer, put the prepped mold box on your work surface, and make sure everything’s clean.
Then get a cold drink, put on old clothes you don’t care about losing, use the bathroom, make a phone call, sneeze, whatever it takes to ensure you’re distraction-free for the next half hour or so. 😉
Put on your gloves and respirator or face mask (DO NOT SKIP THIS STEP). I wear glasses but if you don’t, you need goggles, too. ANY time dry mold mix is being used, you should be wearing safety gear.
- Weigh out the water you’ll need for the mold and put it in the plaster bucket.
- Weigh out the right amount of plaster. Do NOT dump it into the bucket.
- SIFT the plaster into the bucket, making sure it evenly disperses into the water.
- Rotate the bucket a few times to settle the plaster, which should form an island in the middle of the water and slowly develop fissures.
- Let the plaster slake for 3-5 minutes. This step ensures that the mix absorbs the water and begins to hydrate.
- If you need more than one bucket of mold mix, this is the time to start the second bucket. It will slake while you work with the first.
- Lower the jiffy mixer into the water and turn on the drill, slowly at first (go too fast and you’ll get a faceful of plaster). Move the mixer throughout the bucket, ensuring that you reach the entire area.
- Mix for a minimum of 3 minutes (very important).
- Stop the drill, lift it out of the plaster bucket and immerse it in the dirty water bucket. Turn it on high and let it run for 30 seconds or so–this will remove the plaster on the mixer.
You should have a smooth, creamy-looking mixture about the consistency of cooling pudding. If it’s extremely thick, you’ll know to increase the water in the next batch. If it’s more like crepe batter, you need to increase the plaster content.
The age of the plaster can affect the consistency of the mix, and also the set time. So can temperature and humidity–you’ll learn to adjust the exact ratios and process according to the environment.
3. Fill the mold
Once the refractory plaster is mixed, you’re on borrowed time. Don’t rush, but take the plaster bucket over to the mold box, and start pouring.
Do not pour directly onto the top of the model. Instead, aim the stream of mold mix into one corner of the bottom of the mold, and let the level rise naturally up and over the design. I like to pour into each corner in turn, allowing the pours to meet in the middle and rise.
- If you run out of plaster: Keep pouring until everything’s in the mold. If the top isn’t completely covered, take your paint brush and pull plaster over the entire design–this keeps batch lines from forming. Then mix yourself another batch and pour over the top. If you keep your water:plaster ratios and mixing techniques the same, the new plaster will fit the old and you’ll be fine (another reason to weigh and mix with a power drill).
- If plaster starts to leak out of a corner, don’t panic. If the leak is big, move your bucket to the next mold and start pouring that one; the plaster in the leaky box will set in a few minutes, sealing the holes. If the leak is small, ignore it; not enough plaster will escape to cause a problem.
Continue pouring the plaster until you reach the mark on the side of the container, then stop. Pour the remaining mix into the next mold. If that’s your only/last mold, scrape the plaster into a trash bag.
When the plaster bucket’s empty, dunk it in the dirty bucket and scrub it until every bit of plaster is gone. Rinse in the clean bucket.
We mixed the plaster for these molds outside, in about 85-degree weather, and it showed: The mold mix was setting about twice as fast as normal, so fast that it was difficult to settle it into the mold.
I upped the percentage of water in the mix, and mixed for a shorter time, which made it easier to pour the refractory. It also makes the mold weaker, more prone to cracking, and potentially harder to get off the glass. I’m less worried about that with these panels, given the size and thickness of the molds.
4. Settle the refractory
Once the mold is poured you’ve got two jobs: Eliminate bubbles, and wait.
Gently thump the mold up and down on the work surface (GENTLY) to help dislodge trapped bubbles. If your plant parts have lots of texture and undercuts that could potentially trap bubbles, use dip your soft brush into the wet plaster and very gently brush the surface of the mold.
This should force them to rise up in the still-loose plaster, away from what will become the mold surface. (make sure you clean the brush before the plaster sets).
When the mold starts to stiffen, go back and smooth the top. Then, give the mold about an hour to fully set. It’ll be set and solid before that, but if you give it a little extra time it’ll be less fragile.
Clean out the mold
1. Unwrap and trim the mold
Finish uncovering the mold, and trim off all the sharp corners on the outside. Then turn the mold over and check out the clay inside the mold: It’s all got to come out.
Start by trimming the edges of the mold, inside and out. If the clay model wasn’t completely sealed down to the tar paper, the plaster will have filled in at least part of the top (right).
Make your first pass straight down the center, and pull out the first snake, then start removing material on either side.
The object of the game is to hollow out the clay. When enough is removed, the remaining clay should (mostly) peel out of the mold, leaving it clean.
How hollow it needs to be depends on the clay you’re using. Be patient, and try NOT to scrape against the mold itself (below, left).
2. Clean up the mold
Once most of the clay is out, examine the mold. Scrape any protrusions off the sides (left) and soften the contours caused by big air bubbles so that they’re fully open to the mold. You don’t want to leave any thin, fragile areas that could break off and embed into your glass.
Identify areas that need additional cleaning; there may be clay stuck to corners of the mold, or embedded under plant parts. There could be plaster overlapping onto the plant material. This stuff should be removed, but how you do it depends on what’s stuck to which.
Once I’ve gotten as much of the clay out by hand, I hit the mold with a hard spray of water from the hose. That should remove most of the loose plaster and clay, but keep your spraying to a minimum; the mold is still fragile and you don’t want to make it soupy with extra water.
Be careful breaking out plaster that’s strayed onto plant parts if it’s still attached to the mold; you could break off more than you intend, and damage the mold. Use an Xacto knife to carefully outline the area that needs to come off (above, right), scoring the plaster. Now you can go back in and chip it away with the point of your knife and pick it out (left).
3. (Maybe) remove the plant material
The thinner, less textured plant material may come out as you’re removing the clay. Most will remain in the mold, and you’ll need to decide what can be removed and what should be left to burn out. I try to remove as much as possible without damaging mold detail.
Shelby’s thin leaves and stems came out of the mold easily (left); Carla had to do a bit more work to get her iris leaves and lily pods out without breaking anything. As long as you get to them while they’re still green and flexible, most leaves pull out of the plaster without damaging anything.
Chestnuts are often used in natural dying; it was interesting to see how they had dyed the surrounding plaster (left).
Rule of thumb here: If it’s gonna crack the plaster, don’t take it out. Let it burn out in the kiln. Just make sure that there’s no clay between the plant part and the mold.
4. (Optional) Carve the mold
Your decoration doesn’t have to stop with the plant parts; you can carve directly into the plaster to create additional texture. That’s what I did with the wheat panel at left. If you click to enlarge, you’ll see the contoured texture that outlines the wheat stalks.
You don’t need to go very deep into the plaster, only a millimeter or two, just enough to make an impression. Remember that you’re carving a negative space; the lowest point in your cut will be the highest point in the glass.
A tiny wire tool with a flat end will carve squared-off grooves into the plaster that will become flat channels in the glass (as in the wheat panel). A pointed wire tool will produce ridges and furrows.
You can also patch pinholes in the plaster; put a little plaster in a paper cup, mix in just enough water to make a paste. Dab it into the hole until filled.
Let your mold cure
Your cleaned mold needs to cure for at least 48 hours to strengthen the plaster and reduce the chance of mold cracks. You can fill and fire right away…but the glass will often come out of the mold with more issues when the mold hasn’t cured properly.
We can only have “glass class” on the weekends and it was Sunday, so the molds cured a full week. If we’d needed to hold them much longer than that, or if they had to be stored in a warm place (outside in the hot sun or in a room with firing kilns), I would have sealed them into a plastic bag to slow down the drying.
We now had three finished molds curing; in the next post I’ll talk about how we filled them with glass.