Demonstrating the strength of collagen glues

By Michelle Kirk

For a furniture conservation student, hide glue is a staple material in the workshop. However, what's really going on-why are some stronger than others and are how is this physically apparent?

There are times when a need to understand a process or a material calls for experience based learning-to help bridge this gap, carrying out quick tests allows our theoretical learning to emerge as a tangible experience.

In this case, working on an 18th century dining chair, for which traditionally collagen glues were used in the construction, led to a curiosity about the variety and differences in glues. A quick test would also provide more information about which workshop hide glue to use.

The Bloom strength test, developed by O.T Bloom in 1925, is a way to determine the strength in gelatin adhesives. The test is measured in Bloom grams, gB, with a Bloom gelometer-a flat bottomed plunger with a scale. A higher Bloom gram reading indicates more weight is required to depress the gel, and indicates a stronger adhesive.

With our workshop tests however, we used our humble finger to gauge how firm or soft the gelled glues were. Although we knew most of the samples' Bloom gram via information from the suppliers, to fully understand this scientific measure in physical terms, we had to see for ourselves. For the unidentified sample, with no known Bloom gram, this test would also enable us to compare and identify its properties with the other samples.

The preparation of the five glue samples was followed according to recommendations in Rivers & Umney:

  1. 1.4g of dry glue samples were prepared with 10ml of deionised water, and soaked in a closed container until the glues had completely swollen. This took around 12-14 hours for some of the samples.
  2. For the isinglass the water was then removed and a fresh 10ml of deionised water added.
  3. All five glues were placed in a 55°C water bath until they had dissolved.

6 After 1 hour the isinglass sample was removed, and contents strained through a fine silk/nylon material, in this case, stockings material (don't throw out your old tights, ladies!).

7 The five samples were then placed in a 10°C water bath and cooled for 16-18 hours.

8 To test the strength of the glues, fellow students and I compressed the samples with our fingers and compared the differences.

The preparation revealed some interesting observations in itself-the different soaking times, how the dried glue swelled in the water, the rate at which they dissolved in the water bath, their viscosities (important when considering application practicalities) and their different rates of gelling.

Preparation observations:

Kremer Hide Glue 63010 Liberon Rabbit Skin Glue Kremer Salianski Unidentified pearl glue
After 1 hr in cold water soak Granules swollen to twice their size. Pearls almost completely swollen. Almost all granules completely swollen. Flakes were swollen but darker yellow bits (impurities) remained more solid. Pearls almost completely swollen.
After 1hr in 55° water bath Completely dissolved after 30 minutes. Completely dissolved after 30 minutes. Took the longest for all granules to dissolve. Impurities removed when filtered Completely dissolved after 30 minutes.
Gel formation Took less time than Kremer Salianski and the unidentified to gel. Took the full duration to form a gel. Formed a gel the fastest. Quick gel formation Took the full duration to form a gel.

Bloom test results:
Students were invited to poke the sample gels and observe the different physical properties. From most to least firm, these were the results:

Salianski fish bladder glue
Rabbit skin glue
Kremer hide glue 63010
Pearl glue
Liberon pearl glue

In conclusion any quick test can help confirm or deny assumptions and hypotheses about materials, and offer an opportunity to experience the material fully. These understandings also offer and introduce new ideas previously un-thought of, in this case for example, mixing glues to adjust their properties like viscosity, gelling times and strength.

The observed physical properties of a firmer, harder gel successfully demonstrated the Bloom grams of the samples; the higher the gB, the more firm the gel, and high gB relates to a high molecular weight.


Kremer, "63010", (accessed October 27th 2015).

Rivers, S and Umney, N "Plastics and polymers, coatings and binding media, adhesives and consolidants" in Conservation of Furniture, 2003, Butterworth-Heinenman (Oxford), pg 172.

Rivers, S and Umney, N "Principles of consolidation, aesthetic reintegration and coatings" in Conservation of Furniture, 2003, Butterworth-Heinenman (Oxford), Pg. 569.

Schellmann, Nanke C. (2007), "Animal Glues: a review of their key properties relevant to conservation," in Reviews In Conservation, Number 8. Pg. 58.