# Hourglass > Measuring time with falling grains - Date: c. 1338 - Era: Middle Ages - Domain: [Time & Measurement](https://lastcodex.org/en/domains/measure/) - Rebuild stage: III (Crafts & cities) - Difficulty: 2/5 - URL: https://lastcodex.org/en/inventions/hourglass/ An hourglass measures a set interval: dry sand runs from one glass bulb into another through a narrow neck, at a rate fixed by the neck and the grains. The oldest known picture of one is in Ambrogio Lorenzetti's fresco of Good Government in Siena, painted in 1338–1339. ![Hourglass · Measuring time with falling grains](https://lastcodex.org/_astro/plate.DnEdeLS7_19mrbY.webp) ## Legend 1. Upper bulb 2. Neck where the two bulbs meet 3. Lower bulb 4. Dry, sifted sand 5. Frame with two bases and three uprights ## What it is ![A long wooden frame with five turned uprights holding four identical hourglasses side by side, the sand at rest in each lower bulb, a little in the first glass on the left and more in each glass up to the almost full one on the right](https://lastcodex.org/_astro/v3.CFIG0xhA_1N7nS.webp) _Some frames held several glasses with different running times side by side._ An hourglass, or sandglass, is a pair of glass bulbs joined at a narrow neck and held in a frame. Set upright, it lets the sand in the upper bulb run down into the lower one, and the interval ends when the upper bulb is empty. Turning the frame over starts the same interval again. Glasses were made for a quarter of an hour, half an hour, an hour or other durations, and some frames held several glasses with different running times side by side. The frame has a flat base at each end, so the glass stands either way up, and uprights that guard the bulbs while leaving the sand in view. An hourglass does not tell the time of day. It measures how long something lasts: a sermon, a lecture, a cooking time or a watch on board ship. ## How it works ![Two glass bulbs joined at a narrow neck, with a thin stream of sand falling from the upper bulb onto a heap in the lower one](https://lastcodex.org/_astro/v1.DfsE7Ryp_Zo4zfG.webp) _The neck sets the pace: the grains pass it a few at a time and pile up in a cone below._ Sand does not run out like water. In a water clock the flow slows as the vessel empties, because the pressure at the hole depends on the depth of water above it. In an hourglass the flow stays almost steady from start to finish. The rate depends on the width of the neck and the size of the grains, and hardly at all on how much sand lies above. Physicists still debate the exact reason, but friction between the grains and against the glass plays a large part. A wide neck empties the glass quickly and a narrow one slowly. If the opening is only a few grains across, the grains wedge against each other in an arch over it and the flow stops. Damp grains stick together, and dust or a single large grain can block the neck. A good filling is dry, clean and of an even size. As it drains, the surface in the upper bulb sinks into a hollow above the neck, while the grains below build up into a cone. The duration can be estimated beforehand, as physicists can model how fast grains pass through a given neck, but two glasses never run quite alike: the size and shape of the grains, the exact neck and the damp in the air all change the rate. So the maker runs the sand against a reference timer, adds or pours off sand until the glass gives the interval wanted, and only then closes it. Moisture is the main enemy of an hourglass, which is why old glasses were sealed at the joint. ## How to do it ![Two flask-shaped glass phials joined neck to neck, with a pierced brass disc clamped between their mouths and the joint bound with waxed thread across both necks and sealed with red wax](https://lastcodex.org/_astro/v2.CifpUxdC_vTmrg.webp) _The sand runs through the hole of a brass disc held between the two mouths; waxed thread and red wax bind and seal the joint._ Early glasses were made from two bulbs blown separately and joined mouth to mouth around a thin disc pierced in the centre. The joint was sealed with wax and bound tightly with thread. The same method works with two identical small glass phials with smooth, unchipped rims, bought or made by a glassblower. Do not try to blow, cut or heat glass without training, and handle the phials over a soft surface: hot glass burns and broken glass cuts deeply. Wash river or beach sand in several changes of water until the water runs clear, which carries away most of the fine dust. Sieve it while still wet, outdoors: first through a coarse sieve to remove lumps and large grains, then over a fine one, rinsing it so that the rest of the fines goes through. Keep only the grains that pass the first sieve and stay on the second, and dry them completely in the sun. The fine dust of quartz sand damages the lungs, so pour and handle the dry sand gently and outdoors, and if you have to sieve it dry, wear an FFP3 or P3 respirator. Cut the disc from thin brass sheet or stiff card, slightly wider than the mouths, and pierce a clean round hole in its centre with no burr left on the edges. Pour sand into one phial, lay the disc on its mouth and set the second phial upside down on top, holding the joint with a band of paper for the trials. Turn it and time a full run against a clock, then add or remove sand until it gives the interval you want. Check several runs with each end up. When they agree, close the joint for good: coat it with wax, wind linen thread tightly round it and coat the binding again. Melt the wax in a small pot standing in hot water, never over a direct flame, as hot wax burns the skin and can catch fire. Mount the glass in a frame of two wooden discs joined by three uprights, with a pad of felt or leather where the glass touches the wood. Time it again once it is in the frame, and repeat the check from time to time: damp or worn sand changes the duration. ## History and evolution The hourglass has been credited to the ancient Greeks or to an eighth-century monk named Luitprand, but no surviving object or clear text supports either claim. The oldest known picture is in Ambrogio Lorenzetti's fresco of Good Government in the Palazzo Pubblico of Siena, painted in 1338–1339, where the figure of Temperance holds an hourglass. Written records soon follow. In 1345 Thomas de Stetesham, clerk of the English royal ship _La George_, accounted for twelve glass "horologes" and four more of the same kind bought at Sluys, in Flanders. Nicholas Harris Nicolas, who published the account in 1847, took them to be hourglasses. Around 1393 the _Ménagier de Paris_, a household book written by a Paris townsman, gave a recipe for sand "to put in clocks": the slurry left by sawing black marble tombs, boiled in wine, skimmed and dried in the sun nine times over. A sixteenth-century French workshop manuscript makes the powder from lead melted with salt, then ground, sieved and washed. Sailors timed their watches with sandglasses, and preachers set one on the pulpit to time the sermon. In Greenwich, the National Maritime Museum's record describes a seventeenth-century glass that runs for thirty minutes, with its central joint bound and covered in red wax. Later glassmakers blew both bulbs in one piece, which removed the joint. The hourglass came into use in Europe at about the same time as the [mechanical clock](/en/inventions/mechanical-clock/). ## Key facts - Oldest known depiction: Lorenzetti fresco, Siena, 1338–1339 - Oldest known written record: Glass horologes bought for an English royal ship, 1345 - Rate of flow: Set by the neck and the grains, not by the height of sand - Joint of early glasses: Two bulbs blown apart, then bound together and sealed with wax ## Requires - [Glass](https://lastcodex.org/en/inventions/glass/): Melted sand you can see through (c. 2,500 BCE) - [Rope & Cordage](https://lastcodex.org/en/inventions/rope/): Twisted fibres that hold the world (46,000 years ago) - [Bronze](https://lastcodex.org/en/inventions/bronze/): Copper and tin, harder together (c. 3,300 BCE) ## Sources - [Ambrogio Lorenzetti, Allegory of Good Government, 1338–1339](https://artsandculture.google.com/asset/allegory-of-good-government/owH6OsVYalFGMw), Fondazione Musei Senesi / Google Arts & Culture - [The sands of time](https://www.ahsoc.org/blog/the-sands-of-time/), Antiquarian Horological Society - [First hourglass](https://www.guinnessworldrecords.com/world-records/first-hourglass), Guinness World Records - [The Hourglass (Engines of Our Ingenuity, no. 1469, after R. T. Balmer, Technology and Culture, 1978)](https://engines.egr.uh.edu/episode/1469), John H. Lienhard, University of Houston - [A History of the Royal Navy, vol. II (1847): receipt of Thomas de Stetesham, clerk of La George, 1345–1346](https://archive.org/details/ahistoryroyalna01nicogoog), Nicholas Harris Nicolas, Richard Bentley (scan: Internet Archive) - [Le Ménagier de Paris, composé vers 1393 (ed. J. Pichon, 1846): « Pour faire sablon a mettre a orloges »](https://www.gutenberg.org/ebooks/44070), Project Gutenberg - [Making ‘powder for hourglasses’ in the early modern household](https://recipes.hypotheses.org/7502), Stephanie Pope, The Recipes Project - [Zandloper](https://dbnl.org/tekst/_bie001196501_01/_bie001196501_01_0056.php), Biekorf 66 (1965), DBNL - [Powder for Hourglasses, BnF Ms. Fr. 640, fol. 10r](https://makingandknowing-prototype.cul.columbia.edu/index.html%3Fp=1463.html), The Making and Knowing Project, Columbia University - [Hour-glass, AST0073](https://www.rmg.co.uk/collections/objects/rmgc-object-10235), Royal Museums Greenwich, National Maritime Museum - [The granular silo as a continuum plastic flow: the hour-glass vs the clepsydra](https://doi.org/10.1063/1.4757390), L. Staron, P.-Y. Lagrée, S. Popinet, Physics of Fluids 24 (2012) - [Pressure Independence of Granular Flow through an Aperture](https://doi.org/10.1103/PhysRevLett.104.238002), M. A. Aguirre et al., Physical Review Letters 104 (2010) - [Silicosis](https://www.hse.gov.uk/lung-disease/silicosis.htm), Health and Safety Executive (UK) - [Hourglass (How Products Are Made)](https://www.encyclopedia.com/arts/culture-magazines/hourglass), Gillian S. Holmes, Gale / Encyclopedia.com - [Crystalline Silica Dust (information sheet)](https://www.hsa.ie/eng/publications_and_forms/publications/information_sheets/crystalline-silica-dust---information-sheet.pdf), Health and Safety Authority (Ireland) Fact-checked on 2026-10-15. --- The Last Codex · https://lastcodex.org/en/inventions/hourglass/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.