Jar Test Stand

What you're looking at

This is a jar test, the simplest field check of building soil: a tall glass jar of water and soil, shaken and left to stand so the grains settle into bands by size. In the middle, the corked jar sits on a small sliding carriage, held by a ring on a post to its left; a height gauge stands to its right, with a sliding cursor whose arm reaches across to the side of the jar. On the left a water jug stands behind a tray holding three soil tins marked with one, two and three dots; on the right are a sand-glass and, in front of it, a peg board with a sliding time peg, and at the far right a brick mould with a trial block for the next step. You click a tin or the jar to start, drag the peg board to move through time, and drag the gauge cursor up and down; the carriage shakes, the bands grow, the sand-glass runs and pegs pop up as time passes.

What it means

Grains settle by size: in the guide, sand drops in seconds, silt in minutes, and clay over hours to days, while organic bits float. Once the jar has stood about a day, the thickness of each band gives the soil's proportions, so you measure your soil instead of guessing. The guide's starting ranges are roughly 15 to 25 percent clay for cob and about 5 to 15 percent for rammed earth, and the readout checks the settled clay share against both. The three sample soils and the exact settling times are made up for illustration, band height measures volume rather than weight, and a good jar result is only a first screen: drop tests and trial blocks still follow.

Try this

1. Click the middle tin (two dots), or press 2 with the figure focused. The tin slides forward, the carriage shakes the jar, the water goes cloudy, and then time runs on by itself: sand piles up first, silt above it, and a tan clay band last, until the readout gives sand 52, silt 30, clay 18 percent with cob in. It shows the order of settling and that this soil's clay share falls inside the cob range but above the rammed-earth range.
2. Drag along the peg board or sand-glass toward the left and back right, or press Left and Right (Shift for bigger steps, Home and End to jump to 1 second or 2 days). The time peg slides along its slot, pegs appear at 1 s, 10 s, 1 min, 10 min, 1 h, 6 h, 1 day and 2 days, the sand-glass empties its top bulb, and the bands shrink back or build up; until the clay is down the readout names which grains are down, settling or in suspension, and after that it gives the percentages. The board is on a log scale, so seconds, minutes and days all fit on one short slot and you can see that clay takes thousands of times longer than sand.
3. Drag the gauge on the right of the jar up or down, or press Up and Down (Shift for bigger steps). The cursor slides along the gauge bar and its arm points at a height in the jar; for a few seconds the readout says what share of the settled height the cursor is at and which band it is in, or that it is above the settled soil in cloudy or clear water. Reading heights off a scale is how you turn the bands into percentages in a real jar.
4. Click the first tin (one dot) and then the third tin (three dots), or press 1 and 3, letting each one settle. Tin 1 settles to 38 percent clay, outside both ranges; tin 3 settles to 9 percent clay, inside the rammed-earth range but not the cob range. The same test sorts a fat, clay-rich soil from a lean, sandy one, which tells you what to add or which building method to choose.

Reading the readout

The line starts with the time since shaking (s for seconds, min for minutes, h for hours, or days), or the word shaking while the jar is being shaken. While the jar is still settling it then says, for sand, silt and clay (or all grains together), whether they are down (fully settled), settling, or in suspension (still floating in the water). Once the clay is down it gives sand, silt and clay as percentages of the settled height, then cob in or out and rammed earth in or out, meaning whether the clay share sits inside the guide's 15 to 25 percent cob range and 5 to 15 percent rammed-earth range; while you move the gauge it briefly shows the gauge position instead.

What the model uses

• Jar test: put a handful of soil in a jar of water, shake hard and let it stand a day. Gravel and sand settle at the bottom in seconds, silt above in minutes, clay above that in hours to days, and organic matter floats. Measuring the band thicknesses gives the actual proportions.
• Cob mix: roughly 15–25% clay, the rest sand and aggregate, plus straw for tensile strength.
• Rammed earth mix: much leaner than cob, around 5–15% clay, mostly well-graded sand and gravel.
• Clay particles are under 2 microns, which is why clay feels smooth and holds water.
• Mix percentages and dimensions in the chapter are historical starting points, not stamped designs. (repository source policy)
• Never taste soil; inspect organic content with gloves. A successful sample does not establish a safe building: occupied or load-bearing earth work needs a locally proven design and competent engineering. (International Code Council earthen-building provisions)

Settling is drawn as three smooth log-time windows inside the guide's 'seconds, minutes, hours to days'; real soils grade continuously between sand, silt and clay, fine clay can cloud the water for longer, and settled height measures volume rather than mass. This is a code-drawn explanatory model for orientation, not an equipment drawing or a safety procedure; follow the guide's warnings.