Wet Powder News

> To save a fingerprint
By Kjell Carlsson

I believe that most forensic technicians in the forensic work at the crime scene and in the laboratory may sometimes have the following thought. Did we do the right thing now, didn’t the track look like it could be better than what we came up with? Would we use a different method? I think these thoughts can sometimes be in the head of forensics after a job done.

The positive thing about these considerations is that we question whether the methods we use can perhaps be done in a different way. Many years ago, CNA (Cyan Acrylate) was introduced for developing fingerprints. When heated, CNA emits a gas that causes fingerprints deposited on non-absorbent surfaces. Sometimes developed prints are also treated with Basic Yellow40 which can give a better contrast in the track.

Often the result with CNA development is very good, but unfortunately sometimes less good if the track material is exposed to dirt, rain or moisture before track securing.

Fig. 1, 2 and 3 show three different fingerprints on plastic material (plastic bags), which have been developed with CNA and Basic Yellow 40. Before the development, the bags were allowed to lie in the rain for an hour or so. The impression seen in Fig. 1 shows that some parts of the impression are completely missing (spotty coverage). These “spots” lack papillary lines- The two tracks in Fig. 2 and Fig. 3 show similar partial thinnings in the pattern. I think fingerprint examiners recognize these effects. It is likely that a trace exposed to water will have a certain negative impact on the trace quality after DNA development. But a reasonable question is, can really clean rainwater wash away all the substance that is in a fingerprint that was tried to develop with smoke particles from CNA. It is likely that the CNA smoke settles on the surface of the impression and does not go down into the deeper parts of the track.

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Fig 1

Fig 2

                    Fig. 3

Could a so-called color suspension be used for a further development attempt after the DNA development. The logical thing is that such a test should be done with a suspension of small paint particles. Wet Powder White contains titanium dioxide which has significantly smaller particles (about 1/1000 mm) when compared to carbon and iron oxide particles which are common in other suspensions. Thus, the three different tracks 1, 2 and 3 have also passed a “third attempt” with regular Wet Powder White without any special further treatment of the tracks.

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Fig 4

Fig 5


The result is shown in Fig. 4, 5 and 6 (Compare with 1, 2 and 3 respectively). With clarity, the tests show that marks on plastic bags left in the rain and developed with CNA and Basic Yellow 40 with barely usable quality can be “rescued” by a third attempt with Wet Powder White. Wet Powder Black, which is carbon-based and has slightly larger color particles, works worse.

Readers of this article may believe that the favorable results in described samples with Wet Powder White assume that the tracks must first be treated with CNA/Basic Yellow40 to obtain these favorable results. But it’s not. Fig. 7 and 8 show results with similar sample tracks developed only with Wet Powder White.

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Fig 7

Fig 8

In conclusion, the author would like to add the following. Hopefully this topic is not fully developed. Right now, some tests are underway of another variant of Wet Powder with fluorescent color pigments. This could give good results if the fingerprint is deposited on a surface with disturbing variations in print and color. Hopefully this can be finished soon and appear in future issues of the newspaper BEVIS.