**4. Conclusions**

A violet dye extracted from the purple-pink petals of the invasive alien plant *Impatiens glandulifera* Royle can be used as a natural dye with a pigment printing paste to produce screen printing inks up to a concentration of 5 g of the dye per 100 g of printing paste. Higher concentrations of the dye extract impaired the performance of the acrylate-based thickener and consequently lowered the viscosity of the printing ink.

The prints were purple-brown on Japanese knotweed papers and browner on other substrates. Greater absorption of the inks and darker colors were obtained on the fabrics. All prints had very good rubbing fastness, but faded considerably when exposed to light (average grade 3). The wash fastness of the prints on cotton fabric was fair (grade 3) and poor on polyester fabric (grade 2–3), but the prints had good resistance to wet ironing (grade 4).

The addition of acid to the printing paste caused a lighter violet color and the addition of alkali drastically changed the color of the ink to green. Both additives increased the light fastness of the prints, especially on papers for one grade; however, they decreased the wet fastness of the prints on fabrics, on average for one grade.

Due to the excellent adhesion resistance of the printing ink to extreme numbers of rubbing repetitions, the natural dye can be successfully used for printing on paper products that are not directly exposed to light. However, its use on textiles is limited due to its lower wash fastness.

**Funding:** This research was funded by the European research project UIA02-228 APPLAUSE.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable. **Acknowledgments:** The author thanks the project partners; the National Institute of Chemistry (Ljubljana, Slovenia) for the preparation of dye extract and Pulp and Paper Institute (Ljubljana, Slovenia) for the production of Japanese knotweed papers.

**Conflicts of Interest:** The author declares no conflict of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
