The Sticky Issue of Making Paste (Part II)

By Mito Matsumaru, MA Conservation Studies (Alumna, 2018)

In a previous post I described the initial stages of my research in the properties and preparation of wheat starch paste.

I will now continue with an outline description of the quantitative and qualitative tests my paste samples were subjected to in order to identify which parameters might influence the properties of paste.

Testing, testing and more testing

First, each of the 18 successful batches (one of the batches was completely undercooked and could not be used) was separated into 4 samples with different dilution grades (non diluted, 1:0.5 1:1, 1:2 with water).

The resulting 72 samples were then extensively tested for visual, rheological, adhesive and usability properties as captured in the following table.

Most of the properties were evaluated qualitatively in an 'ad-hoc' manner, with the exception of the Rolling Ball Tack Test and Adhesive Strength T-Peel test which followed established quantitative test procedures.

As a picture is worth a thousand words, let me provide a few examples:

Colour was evaluated by visual inspection of the samples. The picture below shows typical examples of the white, cream yellow and blue grey paste samples.

Gloss was likewise evaluated by visual inspection. The picture below shows exampled of matte and shiny paste samples.

Translucence was evaluated by applying a thin even coat of paste on a transparent plastic substrate as demonstrated in the picture below:

Some characteristics like stringiness and finger tack were estimated by touch.

In the case of stringiness, I measured the maximum length of the 'string' created by a drop of paste stretched between two slowly separating fingers. In the case of finger tack, the difficulty of separating fingers after quickly pressing a drop of paste between them was assessed.

Ease of application - also known as 'brushability' - was probably the most subjective of all criteria and was simply measured by brushing samples of paste on a piece of paper and evaluating the homogeneity of coverage and lack of 'bleeding' of paste from the brush when at rest.

In terms of quantitative tests, tack was evaluated according to the ASTM D3121 standard also called the 'rolling ball' tack test. The test simply measures how far a metal ball rolls on a strip of paper coated with paste. To ensure that the ball rolls at a constant speed for all cases, a bespoke brass slope had to be built with the collaboration of West Dean's clock conservation department.

Finally, adhesive strength was measured using the standardised 'T-Peel' test when two strips of paper glued with a sample of paste and left to cure (5 weeks in this case) are slowly pried apart by a tensometer, a device that precisely measures the applied force. This test was made possible thanks to the Mechanical Engineering Laboratory at Imperial College London who kindly gave me access to their equipment.

Results and analysis

Having collected a sizeable amount of data (72 samples corresponding to variations of 5 paste recipe parameters and evaluated across 12 different criteria…72 times 5 times 12 … Pheew!), I used statistical data analysis tools to identify relevant correlations and trends.

To make a very long story short, three parameters appeared to strongly impact the quality of the resulting paste: The cooking temperature, starch to water ratio and sieving process.

Conversely other parameters, such as the source of starch, pre-soaking or the cooking method used, did not show any significant impact on paste properties.

In terms of cooking temperature there was a clear trend for batches cooked under 80°C (of the mixture, not the setting) to have lower tack, cohesiveness and applicability than the others.

These 'low temperature' mixtures had a matte appearance and did not reach the 'shiny' and 'transparent' stages during cooking (see my previous post).

Pastes using lower starch:water ratios such as 1:3 were more challenging to cook and the resulting products had applicability and ease of dilution issues. Pastes using higher starch:water ratios such as 1:10 showed significantly less tack.

Finally sieving (which here meant at least 3 cycles of sieving of the paste after cooking) led to very smooth and 'creamy' textures, and significantly improved the applicability of the samples. In fact, sieving even might improve a 'borderline' product to a 'conservation grade' paste.

As an extreme example, Figure 11 illustrates how both the cooking temperature and sieving impact paste quality. Both pastes were cooked using similar cooking duration and paste water ratio (1:5). The top row shows that a paste cooked at 65 ℃ (top left) has a very matte and uneven appearance, as compared to a paste cooked at 90 ℃ (top right) which has satin and smooth appearance.

These visual indicators are confirmed by the other tests and show that 65 ℃ paste would be unsuitable for use. The bottom row shows the same samples after 3 cycles of sieving. The change in both cases is striking. The sieved 65 ℃ paste is now 'borderline', and for the 90 ℃ one, tack and brushability are significantly improved.

All paste samples staying within these key values/parameters, led to "conservation grade" pastes with similar adhesive and applicability properties.

Conclusion and lessons learned

As a conservation student I have heard many stories recommending or disparaging different approaches to Wheat Starch Paste preparation. My research, however superficial and incomplete, has - I believe - shown many of these to be unfounded.

You don't need to be a witch (or a wizard) to brew a good paste. A good - or at least usable - 'conservation grade' [1] paste can be made independently of the cooking method as long as:

  • The starch to water ratio (w/v) is in the [1:5] to [1:8] range
  • The paste mixture is cooked to at least 80C [2].
  • The product is well sieved and kneaded.

And of course as long as you keep stirring !

[1] I am very aware that what makes a "good paste" can vary quite significantly depending on the materials and type of work to be done. My baseline definition here is a generic paste usable for most common book and paper treatments.

[2] Knowing when your mixture actually reaches 80C might require a few calibration trials with a digital thermometer. If you don't have one, make sure paste mixture reaches "shiny" and "transparent" stages during cooking (see my previous post).

A short presentation of this research was made during the 2018 AGM of the ICON Paper and Book group, and is now available on YouTube. (My presentation starts from 1:07:20 - bear with my strong accent!) The MA thesis (2 volumes) is available at West Dean College, UK.

Mito Matsumaru is now working at Cambridge Colleges' Conservation Consortium as a book conservator. For questions and inquiry for digital thesis, you can email her at: [email protected]