United States Department of Agriculture

Agricultural Marketing Service

Federal Grain Inspection Service

Directive 9180.49 June 19, 2015

Distribution: FGIS, OSP, Industry Originating Office: FMD PPMAB

The U.S. Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, 

national origin, sex, religion, age, disability, political beliefs, and marital or familial status. (Not all prohibited 

bases apply to all programs.) Persons with disabilities who require alternate means for communication of program 

information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and 

TDD).

To file a complaint, write to the USDA, Office of Civil Rights, Room 326-W, 1400 Independence Avenue, SW, 

Washington, DC 20250-9410, or call (202) 720-5964 (voice and TDD). USDA is an equal employment opportunity 

employer.

GRADING AND CERTIFICATION OF GRAIN CONTAINING

DIATOMACEOUS EARTH AND SILICA GEL

Contents

GRADING AND CERTIFICATION OF GRAIN CONTAINING DIATOMACEOUS EARTH AND 

1. PURPOSE ...........................................................................................................................2

2.REPLACEMENT HIGHLIGHTS ............................................................................................2

3. BACKGROUND ...................................................................................................................2

4. POLICY ...............................................................................................................................3

5. FORWARDING SAMPLES TO THE CTL ............................................................................3

6. EXAMINATION AND REPORTING PROCEDURE .............................................................3

7. CERTIFICATION .................................................................................................................4

8. EXAMINATION FEES AND BILLING ..................................................................................5

9. REVIEW INSPECTION .......................................................................................................5

10. DISCLAIMER .....................................................................................................................5

11. QUESTIONS ......................................................................................................................5

APPEARANCE OF DIATOMS UNDER THE MICROSCOPE .................................................6
FGIS Directive 9180.49 June 19, 2015

Page 2

1. PURPOSE

The purpose of this directive is to expand the procedures for grading, testing, and 

certification of diatomaceous earth (DIAT) by providing for the inspection of silica gel, a 

manufactured product very similar to naturally occurring diatomaceous earth. These are 

the two main forms of amorphous (non-crystalline) silica.

2. REPLACEMENT HIGHLIGHTS

This directive is revised to update certification instructions to reflect the current online 

certification system, update diatom images on the attachment, and supersedes 

FGIS Program Directive 9180.49, Grading and Certification of Grain Containing 

Diatomaceous Earth and Silica Gel, dated 4-16-96.

3. BACKGROUND

DIAT consists of siliceous skeletons (diatoms) of unicellular plants of the algae family 

which are recognized by the complex structure of their cell walls. Individual diatoms are 

generally less than .005 inch in size; thus requiring microscopic examination for positive 

identification (see attachment).

DIAT is an EPA and FDA approved natural insecticide and is used to control insects in 

grain. When used on grain, DIAT usually appears as a light gray dust. When applied at 

low levels, DIAT may not be readily visible on grain but the texture of the grain may feel 

coarse compared to grain on which DIAT was not applied. Due to the inability to readily 

differentiate DIAT from potentially toxic crystalline substances with the naked eye, 

official inspection procedures require that grain suspected of containing DIAT be graded 

as U.S. Sample grade, the lowest designated quality grade.

The EPA approved silica gel is a synthetic product generally produced by dissolving 

quartz sand (a form of crystalline silica). It closely resembles DIAT in appearance and 

action. It usually appears as a white dust. The silica particles are of a more uniform size 

and pattern than DIAT.

A consensus of research shows that these two products achieve insect control through 

desiccation. The abrasive diatoms can cut through the thin oily or waxy waterproof outer 

layer of the insect and absorb this oily or waxy (lipid) outer layer by direct contact. Either 

way causes a loss of water within the insect. The insect dries out and dies.
FGIS Directive 9180.49 June 19, 2015

Page 3

4. POLICY

An applicant for inspection who has grain which contains or appears to contain DIAT 

or silica gel and who wishes to have the grain graded as though it did not contain an 

unknown foreign substance, must request a microscopic examination for DIAT or silica 

gel. Upon receipt of the request for examination, the inspector shall forward a sample 

of the grain to the Commodity Testing & Reference Laboratory (CTL) for examination. 

The grade of the grain will not be certificated until the laboratory result is received. Only 

the presence of DIAT or silica gel is being verified; the level of DIAT or silica gel in the 

sample will not be determined.

If a request for the examination of the grain for DIAT or silica gel is not received, and 

the grain at the time of inspection is found to contain an unknown foreign substance, 

even though the substance may be DIAT or silica gel, the grain will be graded as U.S. 

Sample grade. The microscopic examination for the presence of DIAT or silica gel must 

be specifically requested.

5. FORWARDING SAMPLES TO THE CTL

Upon receipt of a request for a DIAT or silica gel examination, send a 100-gram 

representative portion of the original sample to CTL, currently the only approved 

laboratory for official examination. The sample shall be identified by a carrier/sample 

identification number, agency or field office name, location, mailing address, and 

telephone number. Include a note with the sample that a test for DIAT or silica gel (or 

both) is requested. Send the sample(s) to:

Technology & Science Division, CTL 

USDA, AMS, National Grain Center

10383 North Ambassador Dr. 

Kansas City, MO 64153
FGIS Directive 9180.49 June 19, 2015

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6. EXAMINATION AND REPORTING PROCEDURE

Upon receipt of a sample, the testing laboratory shall perform the examination in 

accordance with the following procedure:

a. Place 50 grams of the grain in a 300 ml. Erlenmeyer flask.

b. Add 100 ml. of 95 percent ethyl alcohol.

c. Stopper the flask and shake it vigorously for fifteen seconds.

d. Immediately transfer 0.20 ml. (200 micro liters) of the alcohol solution to a well in a 

clean and dry porcelain spot plate.

e. Allow the alcohol solution to nearly, but not completely, evaporate to dryness.

f. Add 0.20 ml. (200 micro liters) U.S.P. oil of cloves to the residue on the spot plate 

and thoroughly mix the oil and the residue by agitation. (Do not grind the residue 

against the spot plate which can fragment the diatoms.)

g. Transfer a drop of the oil-residue material to a “Bright- Line” 0.1 mm. depth 

counting chamber (haemacytometer) in accordance with instructions furnished with 

the equipment.

h. Using a 430 magnification microscope, examine the oil-residue material to 

determine only the presence of diatoms and diatom fragments. It is not necessary 

to determine the presence of any other foreign substance. A count of diatoms or 

diatom fragments is not required.

i. Upon completion of the examination, the testing laboratory technician shall 

promptly report the applicable results to the laboratory manager as follows:

“Contains DIAT/silica gel” or “Does not contain DIAT/silica gel.”

j. The laboratory manager will promptly notify the inspection office by telephone, fax, 

or otherwise, of the results of the examination as follows:

“Carrier/Sample Identification No. gel.”; or

“Carrier/Sample Identification No. contains diatomaceous earth/silica gel.”; or

“Carrier/Sample Identification No. does not contain diatomaceous earth/silica gel.”

k. If the results are reported orally, they shall be confirmed in writing.
FGIS Directive 9180.49 June 19, 2015

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7. CERTIFICATION

If the examination determines that the grain does not contain an unknown foreign 

substance other than DIAT or silica gel, the grain is graded as though it contained 

no unknown foreign substance, and the following statement should appear in the 

“Remarks” section of the certificate (show the applicable one):

“Grain contains diatomaceous earth/silica gel.”

If it is determined that the grain does not contain DIAT/silica gel but some other 

unknown foreign substance, the grain is considered U.S. Sample grade in accordance 

with the Official U.S. Standards for Grain. Report the reason for U.S. Sample Grade in 

the “Results” section of the certificate and follow normal protocol referenced in Directive 

9060.2, Implementation of the FGIS-FDA Memorandum of Understanding.

8. EXAMINATION FEES AND BILLING

The fees for FGIS testing are covered by the published fee schedule found in section 

800.71 of the regulations. Fees are assessed in quarter-hour increments when an 

hourly rate is applicable. The test typically takes between 15 and 30 minutes to 

complete.

a. Agencies Forwarding Sample for Analysis. The CTL will bill an agency on an hourly 

basis for samples analyzed as part of an original or reinspection service certificated 

by the agency. Agencies may defer this expense to the applicant who requested 

the test for DIAT or silica gel.

b. FGIS Offices Forwarding Samples for Analysis. FGIS field offices will bill the 

applicant at locations which provide original inspection services or when an FGIS 

office provides an appeal inspection service. The hourly fee is applied at locations 

providing original inspection services (including reinspection and appeal inspection 

services). The appropriate appeal inspection unit fee for grade or factor analysis 

is applied when an appeal inspection service is performed on a sample which was 

originally inspected by an agency or when a Board appeal inspection is requested.

9. REVIEW INSPECTION

An examination for the presence of DIAT or silica gel can be performed as part of a 

review inspection (reinspection, appeal, or Board appeal) even though the test was not 

requested as part of the original inspection.

10. DISCLAIMER

This directive does not in any way, either expressed or implied, constitute a 

recommendation or endorsement of the efficacy of DIAT or silica gel, under any label or 

trade name, for any purpose whatsoever
FGIS Directive 9180.49 June 19, 2015

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11. QUESTIONS

Direct any questions concerning this directive to the Policies, Procedures, and Market 

Analysis Branch at (202) 720-0228.

Attachment
June 19, 2015

Page 7

FGIS Directive 9180.49

Attachment

Attachment

APPEARANCE OF DIATOMS UNDER THE MICROSCOPE

TYPICAL DIATOM FORMS – Not excluding other skeletal structures 

(not to scale)

TYPICAL DIATOM FRAGMENTS

(not to scale)