# Blank types

> coarse tests prep, pulp tests assay; order matters

Source: https://geodb.io/guide/topic/qaqc_blank_types_and_placement/ · Category: QAQC
Blanks answer *where contamination happens*, and the two kinds answer
different halves:

- A **coarse (field) blank** — barren, **unpulverised** rock — goes in
  before prep. It tests the crusher and pulveriser, where most
  contamination actually occurs.
- A **pulp (fine) blank** — already pulverised — tests the **assay** step
  and instrument carry-over.

**Placement is the whole point.** A blank after barren samples tests
nothing; insert blanks **after mineralised samples**. Simón's ordering
refinement: after a high-grade interval, **fine blank first, then coarse**
— that way the fine one is *assayed* right after the high-grade sample and
the coarse one is *prepared* right after it. A blank at position 1 of a
batch is equipment cleaning, not QC. (Caveat worth knowing: some labs
alternate two pulverisers, which can defeat placement logic entirely —
Bloom's fix is submitting high-grade samples as a separate batch.)

## Material pitfalls

- *"Bricks are not an acceptable barren material!"* (Bloom)
- **Wrong matrix defeats the assay silently** — limestone in a fire assay
  flux designed for volcanics may under-recover gold, so the blank reads
  clean when it isn't.
- **Too fine** (won't test prep) or **too small** (100 g won't clean a
  1 kg pulveriser bowl).
- **Not verified barren** — Rainy River's local-intrusive blank read >3×
  DL in ~15% of results; marble garden stone brought that to 0.7%.

## In geoDB

geoDB's QC types distinguish field-inserted blanks from lab blanks, and
coarse from pulp where you record it — so log the blank *kind* correctly
when inserting. geoDB does not evaluate placement or material; those are
field-protocol decisions this topic exists to inform.

What geoDB *does* do with a blank once the lab reports it is judge it
against a threshold ladder — see [Blank thresholds](qaqc_blank_thresholds).
Worth knowing here: a blank whose element has no resolvable threshold reads
**"unknown"**, not "pass". A page full of "unknown" blanks is a
configuration gap, not a clean result — and it is not evidence the blank
material or its placement worked.