Little Tree Library Dry Shelves

Little Free Library with new shelves on the kids'/sidewalk side.

The Little Tree Library has a small problem. Sometimes when there's a heavy rain, the rain gets inside. If the books are sitting down on the bottom of the library, they can soak up the water and get damaged. Hardcovers tend to be fine, because their pages are raised up and their covers are apparently able to handle a big of damp, but the paperbacks soak up the moisture and either stick together and mold or bulge out into funny shapes as they dry. I don't mind reading a funny shaped book, but I don't feel that good stewards of the library should let the donations suffer like this. The best solution would be a very slightly raised shelf inside for all the books, preferably one that covers the whole of the library floor. Well, the best solution would be to plug the leak, and we're working on that, too, but we've made adjustments to the top several times and we're pretty confident in the door seals, so a shelf seems like a wise precaution, too.

stack of shelves after bisque firing

We'd been vaguely on the look out for a shelf that would work for a few months, and had a temporary Rubbermaid lid inside the library for a while, but when we took out the lid to clean the library one day, we discovered that there had been moisture collected underneath (and turning brown) for some time. We figured we'd better start looking in earnest for a shelf, but that was about when stay at home order started, so all opportunities for looking for a shelf were at an end. And this isn't really a standard item we're looking for, especially since it can't be too nice or it is likely to get stolen. So I figured the most efficient way to handle it would be to make a shelf (or shelves) out of clay.

underside of a wet clay shelf before firing

Over spring break, I rolled out some slabs, perforated them and added lots of little feet. I used some scrap reclaim clay because they didn't need to be pretty and cut out some holes, more or less at random. I want the shelves perforated for two reasons. One, I don't want water collecting on top of them and two, I wanted to minimize the opportunities for cracking during building, drying, and firing.

stack of bisqued ceramic shelves

I rarely make anything large with slabs and never a big flat shelf like this. My students, on the other hand, often try to make bit flat bases for various sculptures and they usually crack. (I do tell them they are likely to crack, but sometimes students need to try it themselves.) The cracks happen for a number of reasons. The flat slabs don't dry evenly, especially if they are left on a board and have lots of clay built up on top. This tends to mean the top and outside edges dry and shrink before the inside, thus causing cracks as the wet interior tries to shrink against the already dry outside edges. Flat slabs also tend to dry into a curved shape for the same reason. The top dries and shrinks first, pulling the wet bottom up into a slight curve. This can be prevented by drying slabs between boards, or by drying more slowly.

Drying shelves between boards to eliminate warping and cracking

Flat slabs also sometimes have trouble shrinking during firing, because their weight holds them down on the kiln shelf causing cracks during firing. Clay shrinks as it changes from wet to dry and again as it changes from dry clay to fired ceramic during firing. As it shrinks in the kiln, it has to move a bit. Usually this isn't a problem, but a heavy flat slab may have trouble moving across a solid shelf and may therefore crack. Sometimes artists put silica sand down on the shelf so that the slab can shift along the surface as it shrinks, but a foot or concave space under the slab can also help prevent cracks. 

picking up a wet slab of clay with one hand like I am doing to the fired work above, can cause cracks to happen during drying or firing

The cracks that show up during firing can also be from uneven drying or from rough handling before firing. Picking up a wet slab or plate from one edge when it is slightly soft can stress the clay and cause cracks to happen later, during drying or firing. My perforations helped lightened the slab and gave it more airflow, but I also made a point of sandwiching the pieces between two boards when flipping them and while they dried to reduce stress on the slabs and prevent warping.

loading up the new shelves in the little free library, they could certainly stand to be a bit larger

The many, many feet are designed both to distribute the weight of the slab (and books) to mitigate cracking and because the floor of the library is not perfectly flat. I figured lots of little feet can hit the library floor wherever it is highest and the shelf is less likely to wobble. Also I like how the feet look. Of course the main purpose of the little feet is to lift the shelf out of the water if the library leaks again.

books on shelves on the adult/street side of the little free library

I made four shelves which together are a bit small for the library. I fired them just to bisque without any glaze because I figure they are pretty likely to get stolen or broken. Making them more attractive is only likely to bring about their earlier departure or destruction. These shelves were pretty easy to make, so I won't mind too much when I have to remake them, though I'd prefer if it wasn't right away. The only disadvantage of the bisque ware is that it will absorb the water, which could conceivably be a problem if we have a leak followed by freezing weather. If these last long enough to get to freezing weather, I'll consider it a win and glaze the next set.

Measuring Shrinkage

I talk sometimes about measuring my work so that I can account for shrinkage during firing. Clay shrinks during the drying and firing process but metal and plastic bike parts do not shrink, obviously. I need to build my ceramic sculptures a bit larger than I intend them to be when fired. The same issue comes up when building lamps, water fountains, sinks and more lamps.

Bike gear set on top of a fired shrinkage tile. The clay impression of the bike gear is visibly smaller than the gear itself.

At school I tell students we have about 12-13% shrinkage from wet to glaze fired. We tested it quite a while ago, but my clay at home I haven't tested. I suppose I could look up Seattle Pottery's listed shrinkage rate, but I'm firing to a lower temperature and mixing old clay bodies together, so it's best to check shrinkage myself.

Shrinkage tile after firing

The easiest way I know to test shrinkage is to make a shrink tile. I rolled a slab out of my clay (in this case I am using Seamix with sand from Seattle Pottery) and cut it into a rectangle at least 12 centimeters long. I used a ruler and a needle tool to mark a 10 centimeter line in the clay. I labeled the tile with the clay name and punched a hole in case I want to hang the tile later. Then I let it dry slowly before firing to prevent warping. Shrinkage can still be measured accurately with a flexible ruler on a bent tile, but it's easier if the tile stays flat.

Shrinkage tile with ruler to compare measurements before and after

  

I marked each centimeter just in case I made a mistake on one of the marks. With more marks, I can check shrinkage for 10 centimeters or each centimeter individually. (For those of you who enjoy a math challenge, go ahead an use inches for your marks, but calculating shrinkage rates and double checking measurements is much easier in a base ten system.) After firing I remeasured the original line. The new length is 9.4 centimeters. I lost .6 centimeters during firing. My shrinkage rate is 6%.

Metal rod standing inside wet clay form. Notice the 6% gap.

Knowing the shrinkage rate is helpful to give me a sense of how much the clay changes. I could buy or make a shrink rule to help with measurements (we have one in the YVCC clay studio), but my usual approach is to guess and estimate. This week I decided to guess during building and then use the fired shrinkage tile and my ruler to check my guessed measurements. 

Measuring the bike rod against the shrinkage tile measurements.

I built a form into which I will insert a tall black metal rod (the part that holds up the seat on a bicycle). I measured the diameter of the rod using my shrinkage tile. The diameter was about 3.5 marks on the tile, which translates to 3.5 centimeters before firing. (I didn't bother to check the actual diameter of the rod because it doesn't exactly matter using this method.)

Measuring the wet clay opening with standard centimeters

Interestingly, the interior diameter I had built into the form using the highly precise art of guessing was almost exactly 3.5 centimeters. Go me!

Pressing a bike part into the wet clay

For the smaller attachments, I pressed the plastic piece into the wet clay to mark the location and general shape--in this case a circle with a little bit sticking out at the top. Then I measured the plastic on my shrinkage tile and used my ruler to mark the clay with the appropriately enlarged dimensions. The result was very similar to what would have happened if I had pressed the plastic into the clay and wiggled it around a bit (my method for the metal rod) or cut out around the plastic piece using a thick cutting tool.

Impression of the plastic bike part outlined with dashes marking the scaled up measurement 

Six percent shrinkage is almost insignificant at a small scale, but being able to measure correctly becomes more important at a larger scale, such as when I am planning for large gears to be inserted into the clay. I am drying the work slowly, again, since any warping during drying will obviously result in the measurements being inaccurate. After firing I can check the ceramic openings against the actual bicycle parts to make sure my process is efficient.

Round shrinkage tile with impression of bike gear (I made this to have a visual of the size change in relation to a larger bike gear.)

Fitting a new batch of fired work

I fired some more pieces this weekend, including the first base for a pitcher plant piece. The bike fork fits well into and onto the base. The short leg of the fork is damaged (thus it is short) and I may alter the base because of the metal is bent, but the changes aren't absolutely necessary.

bike fork in place (temporarily) on base

Unfortunately, the other area of the base, where I planned to anchor a bike part, appears to have warped or may have been measured incorrectly. I tested the fired fit with a different gear stack than the one I used originally, but I believe the interiors of this type of gear stack are all the same. The original gear stack is hiding in my messy studio somewhere.

gear held in place on wet clay

I can't tell from the wet clay pictures if I am holding the gear in place because it doesn't fit or because it is too heavy for the wet clay. The wet clay should shrink during drying and firing, allowing for about 10% reduction in the size of the piece after firing. A piece that almost fits wet should shrink to fit after firing, though I can't tell if that happened here.

different gear held in place on fired clay (because it won't fit)

The clay attachment is no longer perfectly round, an error I seem to be encountering with some regularity. The piece may have warped during drying or firing--or, again, I might have not accounted for shrinkage very well. Regardless, I should plan for less shrinkage on small elements like this in the future.

the broken bike fork pictured here illustrates my intent, even if I don't plan to use this particular part

I can proceed with the result of this particular error in several ways. If I absolutely want to maintain the original design, I could grind down the exterior of the clay attachment or the interior of the bike part to make up for the small size discrepancy. Or, I could put something inside the attachment which also goes inside the gear. This places the gear farther from the base itself, but still attached in a similar way to the original plan.

broken bike fork holding up the gear that wouldn't fit on its own

Another option is to revise the design, replacing the intended gear with a different, wider gear and a clay end cap. Or I could replace the gears with some other element all together. 

large end cap holding on larger gear

I could attach an old faucet handle

Of course, I don't have to make all these decisions quite yet. My plan is to build the bulk of the work early in my sabbatical, then glaze everything at once, and then put everything together after glazing. If I attach all the bike parts after all the clay parts have been made, I can trade some of the pieces between one sculpture and another. Though this approach is foreign to a pottery, it seems the most reasonable approach for this group of works. I expect to collect some more bike parts before the project is complete. I expect that I will get more of what I already have (more gears, more chains, more of certain small parts), which means I will be able to choose between similar parts later on and may find a slightly larger gear set or a slightly different fit.
 

one of the second batches of pitcher plants, testing fit on the bike fork