Invisible Inks With Household Ingredients:

Mechanisms and Properties

Ben Artin
Northeastern University College of Professional Studies
CMH1011: Chemical Principles 1
Instructor: Patricia Brandl
June 24, 2011

Abstract

Invisible inks — colorless substances usable for writing that can be turned to color using
a chemical process — have both practical value in covert communications and educational
value in chemistry teaching. Publicly available invisible ink recipes are numerous and varied,
but are often published without any elaboration as to their utility and usability. This paper
evaluates approximately a dozen different invisible ink recipes, with focus on those that can
be prepared from common household ingredients. Safety, cost, availability, shelf-life, and dif-
ficulty of use of the different invisible inks are considered. Based on this evaluation, a general
recommendation is made in favor of heat-based inks for casual covert communications and
chemistry education, and precipitation-based inks for more secure covert communications
and chemistry education in a lab setting.
Additionally, two proposed mechanisms for action of invisible inks based on organic acids
and revealed by heating are evaluated. Oxidation of ink by heat is confirmed to play a role
in revealing such inks.

1 Introduction ing mathematics 2 , computer science 3 , quan-

tum physics 4 , and chemistry 5 .
Parents want to know about the activities of In the realm of chemistry, a popular tech-
their children; generals want to know about the nique for covert communication — seen in sci-
activities of their troops. Throughout human ence education 6 , intelligence operations 5 , and
history, on scale from personal to international, mass-market fiction 7 — are invisible inks.
knowledge has been a key ingredient in power. An invisible ink is any substance that can
In struggle to establish and maintain power, be used for writing (typically on paper) and
people often desire to communicate covertly is not easily detected by the naked eye. The
and intercept others’ covert communications. process of rendering the ink visible (ideally by
As a result, numerous technological advances the intended recipient) is known as developing
have been developed primarily to deal with the ink; the apparatus or substance used to de-
covert communication — both to enable it, velop the ink is the developer.
and to intercept it. Recorded accounts of such This paper reviews several substances that
technology go as far back as 1500 BCE 1 , and can be used as invisible inks, as well as
touch nearly every scientific discipline, includ- their corresponding developers, with focus on

1
those that can be manufactured from common species, Ind1 and Ind2 – , of which one can be
household products and other readily available obtained by protonation of the other:
reagents.
Ind1 Ind2 − + H+
1.1 Classification of inks by mechanism
of action and development and those two species have different colors be-
cause their different bond and electronic struc-
Many ink/developer pairs share the same tures result in their having different absorption
underlying chemical mechanism of action (on spectra in visible frequencies of light.
writing surface) and development. Such re- When Ind1 is exposed to an acidic or basic
lated pairs are often similar to each other on environment, the equilibrium concentrations of
several of the evaluation criteria. It is, there- Ind1 and Ind2 – depend on the pH of the envi-
fore, convenient to group their evaluation by ronment, and therefore so does the color of the
mechanism. mixture of Ind1 and Ind2 – . 15
Mechanisms present among the inks and de- Substances containing acids or bases (such
velopers evaluated in this paper include: as citric acid or ammonia) can be used as in-
visible inks and developed by exposing them to
Sugar solutions developed by heat a suitable pH indicator.
To develop a pH-based invisible ink, a pH
Substances containing sugars (such as honey
indicator in aqueous solution is applied both
solution and sugar solution) can be developed
to the marked regions of the writing surface
by heating. When applied to paper and then
and the unmarked ones. The indicator has to
heated, these solutions turn brown 8 .
be chosen so that the ink produces a different
The reaction responsible for this change of indicator color than the unmarked writing sur-
color is presumably caramelization (a non- face does, which depends both on pH of the ink
enzymic reaction that results in browning of and the pH of the writing surface.
sugars 9 ).

Organic acids developed by heat Inorganic salts developed by a

precipitation reaction
Substances containing organic acids (such as
citric acid in lemon juice) can be developed Water-soluble salts can be developed using
by heating. When applied to paper and then a precipitation reaction, if the salt solution is
heated, these inks turn brown 10 . colorless and a colored precipitate can be pro-
The mechanism of action and development duced.
is unclear; two different possibilities are com- The ink, containing an ion (IonI ) in aque-
monly cited — that the organic acid itself ous solution is applied to a writing surface and
browns with heating 11,12 , and that the acid re- allowed to dry. To develop the ink, aqueous
acts with the polymers of the paper, converting solution of a different ion (IonD ) is applied to
them into compounds that brown on exposure the writing surface. If the two ions combine
to heat 13,14 . into a salt (SaltP ) that has low solubility in
water and forms a colored precipitate:
Acids and bases developed by a pH
IonI + IonD SaltP ↓
indicator

pH indicators are substances whose color then the appearance of colored precipitate of
changes on exposure to acidic or basic solu- SaltP on the writing surface makes the writ-
tions. Specifically, an indicator exists as two ing visible.

2
2 Methods ˆ R 22: Harmful if swallowed.

2.1 Ink Evaluation ˆ R 23/25: Toxic by inhalation and if swal-

lowed.
Inks and developers were assessed on the fol-
lowing criteria:
ˆ R 25: Toxic if swallowed.

Availability ˆ R 36/37: Irritating to eyes and respira-

tory system.
Availability of a reagent is determined to be
“common” if it is readily available from com-
ˆ R 36/38: Irritating to eyes and skin.
mon brick-and-mortar and internet stores, such
as grocery stores; and “specialty” if it is only ˆ R 36/37/38: Irritating to eyes, respira-
available from specialty stores, such as garden- tory system and skin.
ing stores.
ˆ R 37/38: Irritating to respiratory system
Cost and skin.
Except where otherwise specified, cost is ˆ R 41: Risk of serious damage to eyes.
based on retail prices during the week of Jan
20–26 2001 at Walgreens and Stop and Shop in ˆ R 46: May cause heritable genetic dam-
Malden, MA. age.

Preparation ˆ R 62: Possible risk of impaired fertility.

Preparation is classified as “simple” if it

involves nothing beyond mixing with water Contrast
at room temperature, “medium” if common
Contrast of the writing was classified as
household processes, such as boiling water, are
“high” if an ink yielded strokes with high
required, and “difficult” if specialized equip-
contrast against the background, “low” if the
ment (other than safety equipment) is required.
strokes had low contrast against the back-
ground, and “invisible” if strokes were com-
Shelf-life pletely invisible.
Shelf-life was estimated based on prior ex-
perience with household products in question, Stroke width
assuming that the ink is not kept refrigerated.
Stroke width of the writing was classified as
Safety “broad” if only broad strokes (applied with a
cotton swab) were discernible, and “thin” if
Risks of inks and developers were deter- thin strokes (applied with a nib pen) were dis-
mined and summarized by the following stan- cernible.
dard 16,17 risk phrases:

ˆ R 8: Contact with combustible material Sharpness

may cause fire.
Sharpness of the writing was classified as
ˆ R 20: Harmful by inhalation. “sharp” if strokes of ink with food dye were
not visibly distorted or blurred by developing,
ˆ R 20/22: Harmful by inhalation and if and “blurred” if they were blurred by develop-
swallowed. ing.

3
2.2 Choice of inks and developers & Shop— 100% Pure Corn Starch, UPC
688267070365)
In order to be useful for development of pH-
based inks, pH indicators have to be available ˆ Sugar, 10 g dissolved in 100 ml H2 O
in aqueous solution. Common aqueous labo- (Domino® Premium Granulated Pure
ratory indicators (such as phenolphthalein and Cane Sugar, UPC 049200045701)
methyl red) are deemed unsafe for consumer
products in various world regions 18 , and there- ˆ Honey, diluted to 20 % by H2 O (Gunter’s
fore are typically only available in specialized Pure Honey, UPC 021273100129)
stores, such as online lab supply stores. They
are toxic, often carcinogenic, and irritants, as ˆ White onion water (filtrate of 50 g
well as being relatively expensive. Such devel- chopped jumbo white onion boiled in
opers were not evaluated. 500 mL H2 O)
A viable alternative to these is preparation
ˆ Lemon juice, diluted to 20 % by H2 O (Si-
of aqueous solution of flavin, an anthocyanin
cilia Lemon Juice, UPC 030849000053)
present in red cabbage 19 . Red cabbage is
commonly and cheaply available in brick-and- ˆ Milk, diluted to 20 % by H2 O (Hood®
mortar stores. An extract of flavin from red Fat Free Milk, UPC 044100169250)
cabbage can be prepared with a simple stove-
top process. Its shelf life is unknown, and it is along with the following developers:
not hazardous beyond risks involved with boil-
ing water on a stove. ˆ pH indicator: Red cabbage water (fil-
Numerous ionic compounds exist that par- trate of 170 g chopped red cabbage boiled
ticipate in precipitation reactions and there- in 500 mL H2 O; Stop & Shop® Fresh
fore could be used as invisible inks, but many Red Cabbage, UPC 02112041383)
of them are not readily available in consumer
ˆ Iodine: Iodine tincture diluted to 5 %
products due to their toxicity (such as cop-
by H2 O (Swan Iodine Tincture, UPC
per(II) sulfate and potassium ferricyanide);
30869385110)
they can typically only be obtained from spe-
cialty online stores. Similarly, numerous ionic ˆ Heat: 20 min in a kitchen oven heated to
compounds exist that can be used as invisible 200 ‰
ink developers, but are also often hazardous,
expensive, and difficult to obtain. Each ink was applied to a piece of acid-free
Such inks and developers (only available paper of brightness 92 and weight 20 lb four
from laboratory supply stores) were not evalu- times:
ated.
The following invisible inks were used: ˆ Using a cotton swab dipped in ink

ˆ White vinegar (Stop & Shop— Distilled ˆ Using a nib pen dipped in ink
White Vinegar, UPC 688267045745)
ˆ Using a cotton swab dipped in ink with
ˆ Clear Ammonia, diluted to 5 % by food coloring added
H2 O (Walgreens Clear Ammonia, UPC
ˆ Using a nib pen dipped in ink with food
049022352773)
coloring added
ˆ Baking soda, 10 g dissolved in 300 ml
H2 O (Guaranteed Value— Baking Soda, Developing a dyed ink allowed for the shape
UPC 688267067570) of the writing to be compared before and af-
ter development, to ascertain how well a given
ˆ Starch, 10 g dissolved in 50 ml H2 O (Stop developer preserves strokes. Comparison of

4
cotton swab application and nib pen applica- 3.3 Evaluation of inks developed by
tion allows determination of how well different precipitation
strokes are preserved.
Food coloring used was McCormick Red All precipitation-based invisible inks are dif-
Food Color (UPC unknown). ficult to obtain; none were directly evaluated.
See Table 2 for partial evaluation of
precipitation-based inks available in specialty
2.3 Investigation of heat-based
stores.
development of inks based on organic
acids
3.4 Evaluation of other inks developed
To investigate two proposed mechanisms for
by liquid developer
heat-based development of inks based on or-
ganic acids, aqueous inks in a glass container One other ink developed by liquid developer
were treated by heat under conditions identi- was evaluated; see Table 3.
cal to those used to develop them on paper.
Because one of the proposed mechanisms re- 3.5 Evaluation of inks developed by
quires a chemical reaction with paper, if it were heat
the only mechanism of heat-based development
of these inks, inks heated in absence of paper Reagents for sugar-based inks (sugar or
would not undergo the same color change. honey) are readily and cheaply available in
common brick-and-mortar stores. Inks are
3 Results quick and easy to prepare, but will spoil within
1-2 weeks, as the sugars are a growth medium
3.1 Mechanism of heat-based for bacteria, fungi, molds, and yeast. Inks and
development of inks based on organic ink reagents are safe in quantities likely to be
acids encountered in household production of invisi-
ble ink, except for risks associated with inhala-
Inks based on milk and lemon juice, when
tion of powdered sugar, and risks of inhalation
applied to paper and heated in an oven at
and ingestion of spoiled ink.
200 ‰, browned in 10–20 min.
Reagents for pH-based inks (such as vinegar
When the same inks were heated under the
and lemon juic) are readily and cheaply avail-
same conditions in a glass dish, they browned
able in common brick-and-mortar stores. Inks
in a similar time period.
are quick and easy to prepare, and generally
keep better than sugar-based inks. Ink and
3.2 Evaluation of inks developed by pH
reagents are often irritants.
indicator
Heat-based development of invisible inks can
Reagents for pH-based inks (such as vine- be performed using an incandescent or halo-
gar and ammonia) are typically readily and gen bulb, an oven, or an iron. All of these can
cheaply available in common brick-and-mortar cause burns if handled improperly. All of them
stores. Inks are quick and easy to prepare, and also run the risk of igniting the paper in the
do not spoil due to their pH. Inks and reagents, process of ink development, which is likely to
by virtue of being acidic or basic, are often cause bodily harm, as well as destroy the secret
toxic and irritants. message.
See Table 1 for detailed evaluation of ph- Detailed evaluation of heat-developed inks is
based inks. listed in Table 4.

5
 Reagent white ammonia baking soda
vinegar
Active ingredient CH3 COOH NH4 OH NaHCO3
Ink Availability common common common
Cost 0.002 USD/ml 0.001 USD/ml 0.002 USD/g
Preparation simple simple simple
Shelf-life indefinite indefinite indefinite
Safety R 20/22, R 36 R 36/37/38 R 36/37/38
Reagent red cabbage extract
Active ingredient flavin
Availability common
Developer Cost 0.004 USD/mL
Preparation medium
Shelf-life unknown
Safety unknown
Contrast Low Invisible Low
Legibility Stroke width Broad — Broad
Sharpness Blurred — Blurred

Table 1: Evaluation of inks developed by pH indicator

Reagent algicidesa copperasb

Active ingredient CuSO4 FeSO4
Availability specialty specialty
Ink Cost 0.013 USD/g 0.002 USD/g
Shelf-life indefinite indefinite
Safety R 25, R 36/37 R 36/38, R 23/25
Reagent washing sodac
Active ingredient Na2 CO3
Precipitate formed CuCO3 FeCO3
Developer Availability common
Cost 0.002 USD/g
Preparation simple
Shelf-life indefinite
Safety R 20/22, R 37/38, R 41
a Such as Crystal Blue Copper Sulfate Smart Crystals, UPC unknown; cost estimate based
on June 24 2011 price on amazon.com
b Such as Hi Yield Copperas Iron Sulfate, UPC unknown; cost estimate based on June 24

2011 price on marshallgrain.com

c Such as Arm & Hammer® All Natural Super Washing Soda, UPC 033200030201

Table 2: Evaluation of inks developed by precipitation

6
 Reagent starch
Active ingredient starch
Availability common
Ink Cost 0.002 USD/g
Shelf-life indefinite
Safety R 36/37
Reagent iodine tincture
Active ingredient I– , I–3
Availability common
Developer Cost 0.13 USD/ml
Preparation simple
Shelf-life indefinite
Safety R 22, R 36/38, R 46, R 62
Contrast Invisible
Legibility Stroke width —
Sharpness —

Table 3: Evaluation of inks developed by other liquid developer

7
 Reagent sugar honey lemon juice onion juice milk
Active ingredient sugars sugars unknown unknown unknown
Availability common common common common common
Ink Cost 0.012 USD/g 0.012 USD/g 0.008 USD/ml 0.002 USD/ml 0.002 USD/ml
Preparation simple simple simple medium simple
Shelf-life weeks weeks months months days
Safety R 20 R 20 R 36/37 R 20
8

Developer Safety R8 R8 R8 R8 R8
Contrast High High Low Low Low
Legibility Stroke width Thin Thin Thin Thin Thin
Sharpness Sharp Sharp Sharp Sharp Sharp

Table 4: Evaluation of inks developed by heat

4 Conclusions life was not evaluated. However, revealing the
writing requires concentrations of the ink and
4.1 Mechanism of development of inks the developer to be matched, so as to produce a
based on organic acids change in indicator color. (For example, when
Inks based on organic acids, when treated by ammonia ink and baking soda ink were devel-
heat, brown in absence of paper; therefore, a oped using a particular flavin solution, only
possible reaction of the acids with the paper is baking soda yielded visible strokes.) Further-
not the only source of browning (although such more, because the developer in aqueous solu-
a reaction may be present); inks’ oxidation by tion has to be applied to the paper, strokes of
heat contributes to the change of color. invisible ink are blurred. As a result, only thick
strokes of invisible inks were typically legible
when developed by flavin.
4.2 Ink recommendations
Readily available inks and developers were Overall, this makes pH-based ink inconve-
found to be so cheap that cost is unlikely to be nient to use, and unsuitable for applications
a concern for any except iodine. that require fine strokes. They do, how-
Overall, precipitation-based invisible inks ever, provide a practical demonstration of well-
are the most difficult to obtain of those evalu- understood chemistry (acids, bases, and pH),
ated. Given that other ink/developer pairs ex- and are therefore useful in education.
ist that are safe, legible, and readily accessible,
precipitation-based invisible inks are unlikely
to hold much practical value in most applica- Heat-based development is readily available
tions. However, as some of these inks can only in many cases, and although potentially haz-
be developed by a small number of uncommon ardous, the hazards can be mitigated by us-
developers, they might be preferable in situ- ing a uniform and controlled heat source (such
ations in which it is important for the covert as a conventional oven, as opposed to a can-
communications to be resistant to attempts to dle light or an incandescent bulb). The inks
reveal them by trying several common devel- it can be used on are safe and common, but
opers. many have limited shelf-life. The development
Another value of precipitation-based inks is process preserves thin strokes very well. Thin
in chemistry education, where they can be used strokes make detection of covert communica-
to associate basic concepts of general chemistry tions harder, because they distort the writing
(substitution reactions and precipitation) with surface much less than broad strokes; therefore,
a practical application (although one of limited heat-based inks are a good choice for covert
use). writing, but would not be suitable where an
Inks developed by iodine are safe and readily adversary is expecting covert communications
available, but iodine stains skin, is toxic, and and therefore likely to try common developers
is less readily available than the inks. Over- such as heat.
all, these inks are less convenient to use than
pH-based or heat-based inks. Mechanisms involved in heat-based develop-
Educationally, iodine-developed inks can be ment of inks are either poorly understood or
interesting as a way of demonstrating impor- chemically complex, and therefore provide a
tant chemical concepts, such as starch-iodine simple demonstration of chemistry that can-
reaction used in redox titration of iodine. not be explained simply. A heat-based invisi-
pH-based invisible inks are readily available, ble ink demonstration might be useful for get-
have a long shelf-life, and are often safe. The ting someone interested in science, but curios-
only readily available aqueous pH indicator ity about the details of the underlying chem-
(flavin) is safe and easy to prepare; its shelf- istry is likely to go unsatisfied.

9
5 Further research None of the precipitation-based inks were
readily available, but it is possible that some
While red cabbage extract is a popular of them can be manufactured from common in-
household source of flavin, there are others gredients using other chemical processes. This
(such as grapes). To make an appropriate rec- was not considered for this paper, but if true
ommendation among the different sources of would make it possible to create more secure
flavin, the extracts should be prepared and inks from household ingredients.
their properties evaluated.

References

[1] David Kahn. The Codebreakers. Scribner, 1996.

[2] Heffrey Hoffstein, Jill Pipher, and J.H. Silverman. An Introduction to Mathematical Cryp-
tography. Springer Verlag, 2008.

[3] Arto Salomaa. Public-Key Cryptography. Springer Verlag, 1996.

[4] Anton Zeilinger, Artur K. Ekert, and Dirk Bouwmeester. The Physics of Quantum Informa-
tion: Quantum Cryptography, Quantum Teleportation, Quantum Computation. Springer
Verlag, 2000.

[5] June B. Bell. Invisible-ink Formula Stays Invisible. National Law Journal, 24(26), March
2002.

[6] Trevor Cook. Experiments with States of Matter. PowerKids Press, 2009.

[7] Joanne K. Rowling. Prisoner of Azkaban. Bloomsbury Publishing PLC, 1999.

[8] Robert Gardner and Barbara Gardner Conklin. Chemistry Science Fair Projects: Using
French Fries, Gumdrops, Soap, and Other Organic Stuff. Enslow Publishers, 2004.

[9] M. del Pilar Buera, Jorge Chirife, Silvia L. Resnik, and Roberto D. Lozano. Nonenzymatic
browning in liquid model systems of high water activity: Kinetics of color changes due to
caramelization of various single sugars. Journal of Food Science, 52(4):1059–1062, 1987.

[10] Cy Tymony. Sneakier Uses for Everyday Things. Andrews McMeel Publishing, 2005.

[11] Anne Marie Helmenstine. Lemon Juice Invisible Ink, . URL http://www.topspysecrets.
com/lemon-juice-invisible-ink.html.

[12] Create Invisible Ink With Lemon Juice — Fun Science Experiments for Kids, . URL
http://www.sciencekids.co.nz/experiments/invisibleink.html.

[13] Invisible Ink — Wikipedia, the Free Encyclopedia, . URL http://en.wikipedia.org/

wiki/Invisible_ink.

[14] Anne Marie Helmenstine. Make Invisible Ink — Lemon Juice, . URL http://chemistry.
about.com/cs/howtos/ht/invisibleink3.htm.

[15] Daniel C. Harris. Exploring Chemical Analysis. W H Freeman Co, 2004.

10
[16] Commission Directive 2001/59/EC of 6 August 2001: Adapting to Technical Progress for
the 28th time Council Directive 67/548/EEC on the Approximation of the Laws, Regula-
tions and Administrative Provisions Relating to the Classification, Packaging and Labelling
of Dangerous Substances, . URL http://eur-lex.europa.eu/LexUriServ/LexUriServ.
do?uri=CELEX:32001L0059:EN:HTML.

[17] List of R-phrases — Wikipedia, the Free Encyclopedia, . URL http://en.wikipedia.

org/wiki/List_of_R-phrases.

[18] FDA Proposes Ban on OTC Sale of Laxative Ingredient, 1997. URL http://archive.
hhs.gov/news/press/1997pres/970829.html.

[19] New scientist. 191(2558-2566):65, 2006.

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