Making a Tiki Mug

Mark Boszko edited this page May 22, 2016 · 8 revisions

My goal is to have made at least one tiki mug of my own, for Kilauea Cove, by the end of 2016. How am I going to do that? I don’t know yet! I haven't made anything with clay since I was in school, probably 25+ years ago. But I am researching and slowly procuring the tools and materials.

I’m inspired by TikiRob to test some small tiles or medallions first, to get my head around the process of sculpting, and to test some different glazes. Here's what I've learned so far:

Step-by-step

Based on Wendy Cevola’s “list of steps for a mug that is made more than once”, edited and annotated by me, often with Wendy's own notes from elsewhere on TC, usually from her master thread in “Other Crafts”.

Design your mug

  • Sketch it out on paper before turning to clay

Sculpt the mug

  • Wendy uses Roma Plastilina Clay No. 4, which is a hard, oil-based clay. No. 4 is their extra-hard version.[^1] It contains sulfur, so it may not be suitable for molding in silicone, if you might use it to create things other than mugs.
  • I have seen other artists on Tiki Central use Monster Clay, which is oil-and-wax-based. When at room temperature, the wax makes it harder than usual oil-based clays, to hold detail better. They also have a “hard” grade, and Monster Clay is sulfur-free, so you can also mold it with platinum silicone if you're making resin duplicates (for statues, pendants, etc.).[^2]
  • These harder clays will likely require you to heat it, if you want it soft enough to work with your hands. Monster Clay can be microwaved in its shipping tub (beware the molten center!), or they can be heated with a heat gun or incandescent lamp.
  • At room temperature, the hardness means that precision tools can be used to capture smaller details. It should also stand up better to the rigors of plaster molding.
  • Sculpt larger than you want the finished piece to be, since the cast piece will shrink while drying, again at the bisque fire, and again during the final firing with glaze. I've seen figures from various places that put shrinkage around 10–12%.[^3]

[^1]: As of 2016-05-17, it looks like they may be discontinuing this hardness? The white version of the clay is on clearance at the manufacturer's site.

[^2]: I've bought some Monster Clay to start with, but I got the medium hardness, which may have been a mistake, after reading more of Wendy's posts.

[^3]: I assume this would depend on what your slip recipe is?

Make the plaster mold.

  • (There is more about mold-making in Wendy's master thread.)
  • (On p.114–115, Wendy bisque fires the master before making the mold. Not sure why. Advantages? (Maybe that it's harder to damage during mold-making?) Circumstances in which it's recommended?)
  • Use mold soap on the sculpt and the exposed plaster areas when pouring the next part of the mold.
  • Pouring the plaster, Wendy likes to dip her fingers in, and up the surface of the sculpt, to make sure there are no bubbles
  • Clean mold soap off of the mold, with a soft sponge.
  • Let the mold dry out completely before use. This can take weeks unless you use a fan, and have the molds raised for circulation, or rotate the exposed sides frequently.
  • Making a new mold from an old mold, on p.97 of Wendy's thread. (Not much in the way of details.)

Prepare the slip (liquid clay)

  • We're assuming using pre-made slip, that comes in large jars. You can make your own from powdered clay and water, or other custom recipes (see the Essential book below), but when first starting out, pre-mixed slip may be the most pragmatic and dependable option.
  • Stir the slip, because the clay particles may have settled a bit. Use a mixer attachment for a drill, and only turn it on once it's in the bottom of the container. Do not allow a whirlpool to form in the surface, which will draw in air and trap bubbles, which may lead to defects later.
  • To prevent defects, first pour the slip through a screen to remove any chunks.

Prepare the mold

  • First, spray the interior mold walls with water, to encourage the start of capillary action.
  • Assemble the mold pieces, and secure straps around the perimeter.
  • If there is any question about it being loose or moving at all, you can use wooden shims under the straps to tighten the mold further.

Cast slip into the mold

  • If the opening to the mold is small, you may need to use a funnel.
  • Pour slowly, and hit the side of the mold with a rubber mallet, to release any bubbles
  • Pour for about 10 seconds, and then stop to check for leaks. Quickly lower each side of the mold, so that will slip will flow along the mold seams. That way, if there is a leak, you may find it before there is much more pressure from the weight of the slip in a full mold. This will also quickly coat all sides with a thin coat of slip, and may encourage the slip along the seams to set up, preventing minor leaks.
  • If the mold leaks, you may be able to plug up the leak with clay
  • Quickly pour in the rest of the slip.
  • Over the first 15 minutes or so, continue to top off the slips in the mold, as the water is absorbed by the plaster. (This ensures that the walls at the top of the cast will be as thick as the rest.)
  • On the first pour, wait an hour for the slip to build up a wall of clay on the interior of the mold. Once you see how thick the walls come out, you can determine if you need to leave the slip in the mold for a longer or shorter time on subsequent pours. (The Essential book recommends only 15 minutes for a cast before pouring out excess slip, but Wendy has mentioned several times that she likes thick walls and heavy mugs. This also may be because she's doing further sculpting on the Bob-style mugs after casting. Experiment?) Wendy suggests (p.102) that you write the time required for the wall thickness you like on the mold itself. “Notes on molds are a big help, especially if you pull out an old one to use.”
  • Turn the mold on its side and pour out the excess slip. If the opening is small, and the mold interior is large, you may need to pour very slowly, so as to avoid the clay filling the entire opening, which will create a vacuum seal. The vacuum will pull on the interior of the newly-deposited clay walls and deform your cast. This is what Wendy refers to as the “glug-glug effect.” A pump (Wendy uses a gasoline siphon) can remove the bulk of the slip, and avoid this problem.
  • Wendy checks inside with a flashlight to make sure that the cast was not distorted during draining.
  • Let the mold sit upside down overnight, to completely drain.

Remove the mug from the mold

  • You may need to cut the clay away from the pour spout before opening the mold, to avoid tearing the top of the cast mug when you disassemble the mold.
  • Remove the straps.
  • Open the mold and take out the mug.
  • If there's any slip stuck of the mold, let it dry a bit, and then you can just pop it off.
  • Let the molds dry in the sun before casting again. Depending on the size of the cast, this could be overnight, or you could be able to pull several pieces from the same mold over a single day.
  • This is now “greenware”

Clean the mug

  • Cut off and smooth all mold joint lines.
  • You may need to clean up detail on the mug if the sculpt had any undercuts or very fine detail.
  • If there is small detail and/or pieces that require undercuts, this may be the time to add those. (See the wahine's hair flower on Wendy’s Zombie Flame mugs.)
  • If there are any bubbles in the surface, repair with liquid slip and excess cut slip from old casts. (Keep slip cuttings in a plastic bag, to keep moist.
  • If you need to repair or glue a detail on, cover with plastic for a few days, so that the moisture will equalize in the piece.
  • GROG recommends taping a piece of sand paper to a flat surface, and moving the mug around in a circular motion on top, to ensure a flat bottom to the mug so they don't wobble.

Dry the mug

  • Let it dry until all the moisture is out of the clay. In the winter that can be a couple of weeks. The mug will shrink during the drying.
  • If the mug develops a crack while drying, try Marx Magic Mender for repairs. This can also be used for repairing air bubbles. (MadDogMike on TC recommended this.)
  • Another repair option is “paper clay,” which GROG recommends. [Seems like the fiber makes it less susceptible to shrinkage?] On p.101, Wendy makes a reference that Magic Mender is may actually be paper clay, in a prepared form?
  • Sand the mold lines and fine tune the detail on the mug. Add name and number on the bottom.
  • Use an air compressor to blow off all dust inside and out.

Bisque Fire

  • Seems like in some situations, there might be a reason to pre-heat the mugs in the kiln to bake out any excess water before the actual firing.
  • It’s okay to let the greenware touch in the kiln for the bisque fire.
  • Fire the mug to bisque, and let cool in kiln for 24 hours after the kiln is turned off.
  • The mug will again shrink during this first firing.
  • When it is cooled, remove it from kiln
  • You may need to re-sand mold lines at this point, as the shrinkage may sharpen any bit of line that was left previously.
  • Hit it with a high force hose to remove any dust that the air compressor missed.
  • Let the mug sit for a couple of days to again dry out. Protect it from dust during this time. One spec of dust, and you will have the glaze crawl and leave an unglazed area.
  • If there are cracks at this point, you will need to repair it (with Magic Mender?) before glazing. [Does Magic Mender require a second bisque fire, or can you go right to glaze?]

Glaze

  • When dry, paint the mug with three layers of glaze letting it dry in-between each layer. (It appears that Wendy does three coats for color density. YMMV?)
  • If you are using under-glaze there will be a fourth layer of clear glaze applied on top.
  • Wendy sometimes uses a paint sprayer to paint glaze on the mugs, especially if she's trying to do a color gradient. (See Zombie Torch, Hawaiian Hut crawl mug) [technique?]

High(er?) Fire

  • When all the layers of glaze have dried then load the kiln and fire over night again. Let cool for at least 24 hours.
  • Kiln wash on the shelf to prevent glaze sticking? How does this work?
  • You can’t let pieces touch when firing the glaze. Basically the glaze becomes molten glass while in the kiln, and the pieces would fuse together. You also need to be aware of glaze fusing to the kiln shelves. Items with glaze on the bottom will need to go on spikes. [What is kiln wash? Does that go here?]
  • When you remove them from the kiln, check for skips. If there are none, you are done. If there are skips, then you scrub the area, let it dry, and do the whole glazing process on that one spot again, including re-firing.
  • If you are glazing a pendant, don't forget to clean out the holes for the necklace.

After-fire enamel

  • If you have glazed bottoms that fire on spikes, you may need to sand down the areas where the spikes touched the glaze (GROG's sandpaper-taped-to-table trick can help here), and repair with matching enamel paint.
  • You may also want to use enamel for other small details that will not work in glaze (since everything melts into everything else, fine detail will probably not be retained)
  • Enamel paint can then be cured in a normal household oven. This is hot enough to cure the enamel, but not hot enough to re-melt the glaze. [technique?]

Resources

Clay Basics

Questions to research:

  • What kind of clay should I be using? for sculpting? for slip?
  • What are cones? What is the firing and glazing process?

Sculpting

  • What tools do I need?
  • Are there things I should know about designing my sculpt so it doesn't slump in the kiln?

Mold-making and Casting

  • The Essential Guide to Mold Making & Slip Casting (A Lark Ceramics Book) by by Andrew Martin. As of this writing (2016-05-15), seems like it may be temporarily out of print, but I was able to borrow it from my library. (Check your library on WorldCat.) Many people reference this book as the “bible” on slip casting, but while it contains a lot of useful information, it's arranged rather haphazardly, and has this attitude about “real art” that may be off-putting. Also, sections of the book are rather may be a bit confusing if you haven’t worked with clay before, since the author uses jargon without explaining it.

Glaze

Questions:

  • Do I have to make all of my own sample tiles with glaze? Can I order a set of sample chips or something?
  • What is under-glaze vs. regular glaze?
    • Wendy’s answer from p.101: “glaze turns to glass and moves as it melts when fired. That's what's bad about it. Under-glaze is more like a powder in water. It can be built up and layered and when it is fired it stays put. The only movement would be in the clear glaze used to cover it. Picture the Trader Vic's mugs with so many colors. That's under glaze. As long as you do all three layers what is under it [referring to the dots and ghosts painted over the black background on the Pac-Mac Bob] will not show. I've used it on the entire Pac Man Bob and the Bob with leaves.”

Artists

Seattle Pottery Supply 2016-05-21

  • I rented a car and drove down to Seattle Pottery Supply, to try to get my brain around slip casting materials. It was a little overwhelming. After asking a couple of other employees, eventually one lady there, more experienced in slip casting, gave me some pointers to get going. (Sadly, I didn’t not catch her name, or I would thank her again here. That said, everyone at SPS was very helpful.)
  • She recommended creating my master sculpt with water-based clay, because the wax and oil from the Monster Clay (that I was going to use) will clog the pores in the plaster mold, causing it to take much longer to soak up the water when casting. Seems like the speed of casting isn't really a critical thing at this point in my learning (I've certainly seen other people on Tiki Central use oil-based clays for this), but I'll give it a shot. Sculpting with water-based clay seems different enough from the oil-based that I was using, that I suppose I should get used to doing it “the right way.”
  • For this same reason, she recommended against using mold soap on the master positive. Only if there’s a multi-part mold, and then only on the plaster-to-plaster seams. While the mold soap can be washed off to some degree, some will always remain in the plaster pores and cause extended slip casting times.
  • She recommended working in stoneware slip for the tiki mugs, as they will chip less easily than earthenware. She sold me their “Swan” casting slip, which fires at cone 04-6 (I assume that means cone 04 for bisque and cone 6 for glaze (glost?) fire? I should have asked.)
  • She also recommended National Artcraft Co. Casting Rings for determining when to dump the slip from the mold. These rings come in 7 different thicknesses (she recommended #4 for mugs). You set the ring on top of the plaster mold (number side up), just next to the pour hole, and fill the ring with slip. When the button of slip inside the ring becomes leather hard, the mold is ready to dump. She said differences in humidity and plaster density (not to mention moisture from casting multiple items in a row from the same mold) can change the amount of time required for the slip to build to the thickness you desire, so this “button” method is more reliable than setting a timer.
  • Once I get to doing multiple castings at once, she gave me this tip: instead of buying many copies of these $5 casting ring sets, find washers at the hardware store with the same interior diameter, and glue them together to match the height of the ring number I want to pour to. All you basically need is a little ring-shaped dam to keep the test slip on top of the mold.
    • The rings seem to be in roughly 1.5mm increments, with a 14.5mm average interior diameter. (All of the rings are a little sloped so they have a larger diameter at the bottom; the shortest ring has a 14mm inner diameter.) Ring #4 is 6.05mm tall, according to my micrometer. That means two glued-together washers of USS size 1/2" (which actually have an inside diameter of 9/16" / 14.29mm) will be close, if a little thin (5.54mm height for two washers). Maybe the glue will make up the difference. ;-)
  • I picked a single glaze for the test medallions: Duncan RG722 Sea Glass, which says “fire to cone 5-6”. The sample there showed it coming out as this aqua green sand-tumbled Coke bottle kind of finish, though a bit more glossy. Should look nice on what I have planned for my test medallion. She recommended a fan brush with a rounded ferrule to evenly distribute the glaze, if you're not doing small areas of different glazes. Water cleanup. “Everything’s water cleanup with clay,” she said.
  • I also grabbed a couple of Kemper sculpting tools, including one with soft rubber tips, which seemed useful for smoothing the edges of relief elements onto a mug/medallion surface. These wood handles are way nicer than the unfinished ones that came in my $10 beginner’s kit from Amazon. I already feel the Kemper lust forming in my heart.
  • For plaster mixing, she recommended a Jiffy Mixer drill attachment, which she said would help prevent air bubbles from getting into the plaster. “Submerge mixer into materials to be mixed before starting motor; shut motor off before allowing mixing unit to reach surface of the mixed materials.”

Other Art

  • Outdoor volcano with real fire by Choptop

  • Ask Wendy if there are working YouTube links for the PBS segments on their collection? The ones posted on p90 (or so) of her thread are now dead.

  • Poring ahead and stockpiling did not work. Even wrapped in plastic, everything dried out. But, MadDogMike posts: “Wendy, I don't know if space will permit but here's an idea for your precast forms. Wrap them in plastic like you did and place them in a sealed 5 gallon plastic bucket with 1/4 inch of water on the bottom. You'll need to find something to set the mugs/bowls on so they don't sit in the water and check the water lever every month or so. I have odd bags of clay I have stored that way for a couple of years and they don't dry out at all.”