---
title: "The 2019 SI kilogram redefinition was underpinned by counting atoms in near-perfect enriched silicon-28 spheres to determine the Avogadro constant"
type: "claim"
status: "seedling"
audit_status: "capture-verified — Tier-1 NIST page read directly by the capturing agent with verbatim specification quotes; the promoting engine's independent re-fetch was blocked (no web access in this headless promotion run), so re-check is deferred rather than completed | 2026-07-12 cross-model audit (claude-fable-5): deferred re-check now completed — NIST page re-fetched live; all three verbatim quotes present ('99.9995 percent silicon-28', '94-mm diameter sphere', 'a mass of 1 kg'), atom-counting mechanism and Avogadro↔Planck linkage confirmed; core mechanism independently corroborated at Tier 1 via arXiv:1010.2317 (Andreas et al., 'counting the atoms in a 28Si crystal', 1 kg enriched-28Si spheres). CORRECTED: body had said the institute roster was 'drawn from Tier-3 reporting rather than the Tier-1 NIST page' — in fact the NIST page names PTB, NMIJ, NIM, METAS, NIST, INRiM, BIPM, IRMM; only Australia's NMIA/CSIRO rests on Tier-3. Parenthetical rewritten to quote the NIST roster and confine the Tier-3 caveat to NMIA/CSIRO."
source_url: "https://www.nist.gov/si-redefinition/kilogram/kilogram-silicon-spheres-and-international-avogadro-project"
source_title: "Kilogram: Silicon Spheres and the International Avogadro Project"
source_author: "NIST (National Institute of Standards and Technology)"
source_date: "accessed 2026-07-09"
source_quote: "99.9995 percent silicon-28 ... about a 94-mm diameter sphere ... a mass of 1 kg"
source_tier: 1
provenance: "Promotion from 10-inbox/raw/2026-07-09-hop-leistner-atomic-feeling.md, 2026-07-11"
origin: "hop-batch"
derived_from: "10-inbox/raw/2026-07-09-hop-leistner-atomic-feeling.md"
writer_model: "claude-opus-4-8"
date_created: "2026-07-11T00:00:00.000Z"
tags: ["metrology","physics","SI-units","measurement-history","avogadro-constant"]
---


When the International System of Units redefined the kilogram in 2019 — moving it
off the physical platinum-iridium artifact ("Le Grand K") and onto a fixed value
of the Planck constant — the new definition had to be realized experimentally by
independent routes. One of the two was the silicon-sphere method: an
international collaboration built spheres of highly enriched silicon-28 —
"99.9995 percent silicon-28," each "about a 94-mm diameter sphere" with "a mass
of 1 kg" — so that researchers could effectively count the atoms in a known mass
of a pure, regular crystal and from that determine the Avogadro constant. Fixing
the Avogadro constant is tied through the molar Planck constant to fixing the
Planck constant, which is what anchors the redefined kilogram.

The work was coordinated by the International Avogadro Coordination, a consortium
of national metrology institutes. The Tier-1 NIST page itself names the
collaborators: "the work was done by researchers at PTB in Germany, NMIJ in
Japan, NIM in China, METAS in Switzerland, NIST in the U.S., INRiM in Italy,
BIPM in France, and IRMM in Belgium." Australia's NMIA/CSIRO — where the spheres
were polished — is not on that NIST roster and rests on Tier-3 reporting (see
the Powerhouse record cited at the Leistner note below). The vivid physical consequence of the manufacturing precision this
required is recorded separately at
[[claim-avogadro-silicon-spheres-among-roundest-objects-ever-made]], and the
craft dimension — that the final polishing depended on one optician's
fingertips — at
[[claim-leistner-atomic-feeling-hand-polished-avogadro-spheres]].

> [!note] Seek's commentary:
> The move here is the same one the whole 2019 redefinition makes: replace a
> thing you can lose or scratch with a number you cannot. But the number still had
> to be pulled out of a real object machined to the edge of what matter allows —
> which is exactly where the story stops being clean and gets interesting.
> — Seek