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Published Aug 18, 2014

Painting a repeating pattern on a round vessel presents challenges. To be convincing, the pattern needs to expand proportionally with the roundness of the pot. Tony Merino wanted to do this, but really wasn't too excited about revisiting high-school trigonometry class. So he set out to find an easier way, and he did.

In today's post, an excerpt from the September/October 2014 issue of Pottery Making Illustrated , he (and co-author Pam Luke) share the process.- Jennifer Poellot Harnetty, editor

About 15 years ago, I needed to figure out how to paint patterns on pots. This created a dilemma for two reasons. First, patterns require grids and second, most pots are spherical. Mapping a grid on a round surface is difficult.

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To do it correctly, both the width and the height of the columns and rows need to expand proportionally with the diameter of the pot. The obvious solution would be to figure out how to calculate the expansion and measure out the grid. The drawback of this solution? Revisiting trigonometry and calculus. I didn’t want to do that, so I had to learn to cheat. I figured out a way to put a grid on a round surface using a modicum of math ability and some very simple tools. This process requires paper, a 360° protractor, an X-Acto knife; water-based magic markers, and a drafting template.

### Make a Template for the Pattern’s First Line

Place the protractor on a sheet of paper. The heavier the stock of paper, the better.Using a pen, outline the circle on the piece of paper. Divide the circle into the number of segments you want on the pot. Next, place a mark at each segment on the piece of paper. Finally, using the X-Acto knife, remove the circle from the center of the paper. The remainder of the sheet becomes the template with which you will lay out the columns for your pot. Most 360° protractors are only 6 inches in diameter. If you need a larger template, simply mark the center of the circle, draw a line using the center and each of the marks you placed on the outside of the circle. This line should extend past the circumference. Simply draw a larger circle using a compass.

### Transfer the Template to the Form

Place this template over the neck and on to the shoulder of your bisque ware. Try to center the template as accurately as possible. Next, using water-based marker, place a mark on your pot that aligns with each of the marks on your template (figure 1). I have found that water-based markers work best for two reasons. First, they burn out when fired and second, they don’t act as a resist when you begin to paint your pattern on the piece. After this step is complete, you’ll have a circle of evenly spaced dots around the top of your piece.

### Laying Out the Pattern’s Grid

In order to lay out the grid, you’ll need a drafting template, which is a flexible plastic sheet containing a collection of incrementally increasing circular holes. Each of the circles is segmented into quarters with marks at (0°, 90°, 180°, & 270°). Take the drafting template and align one of the quarter section marks with one of the marks you placed on the pot. This will be considered 0°. Align the opposite quarter section mark horizontally with the next mark in the top row that you placed on the piece. This is considered 180°. Next, place two marker dots on the pot, one that aligns with the 90° segment mark on the template, and one that aligns with the 270° segment mark (see figure 2). Repeat this with every segment of your pot. When you finish marking, you will have created a second row of dots on your pot. Repeat this process using the rows of dots you just placed on your pot. As you do this, you’ll notice that the dots in this row are spaced slightly further apart than the first row. In order to get an accurate spacing, you’ll have to go to one of the larger circles in the drafting template to put down the next line of dots (figure 2). Repeat this process as you work up and down the pot. As the pot expands, you’ll need to find a circle on the drafting template with a diameter that’s as wide as the spacing of the dots on your pot. When you are done, you’ll have a pot with an evenly laid out grid of diamonds (figure 3).

### Changing from a Diamond to a Square Grid

The grid you completed in step three should appear to be comprised of diamond shapes. There are two ways to change this into a grid of squares. First, you can simply skip a row and column. Every other mark will should align vertically. Using a slightly darker marker, put an additional dot after every other dot (figure 4). The one thing you have to consider is that for example, if you are going to skip over every other segment, you will need to ensure that the pot is divided into segments divisible by 4, (4, 8, 12, 16, and 20). An easy pattern is one that is similar to the sole of a Birkenstocks shoe, which is created by alternating the orientation of each quarter of your circle. The curve of 1° to 90° and 181° to 270° should alternate. The curve between the 91° to 189° and the 271° to 360° quarters should be directed toward the inside of the circle.

To see more images from the article and to learn about several other variations of patterns as well as glazing options, be sure to read the full article by Anthony Merino and Pam Luke from the September/ October 2014 issue of Pottery Making Illustrated.