TITLE: High resolution negative silicone rubber impressions of perforation on stone personal elements for technological studies
AUTHORS: Stefano Viola, Matteo Gios, Umberto Tecchiati, Stefano Viola

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><div class = "justify" style = "text-align:justify">Although several scientific papers mention the use of HD silicone based impression materials for the study of small perforations, until today there are no published explicit and detailed protocols in this research's field. This technique is particularly useful in the field of the technological study of prehistoric ornaments, in particular for drilling methods. Unfortunately, the originals are often not suitable to be analysed with the main high magnification methods (they are delicate objects; they cannot be moved; they have several hidden points for direct observation) and require the development of a specific study methodology.To explain the method in detail, we propose a procedure divided into 11 phases, and some intermediate steps, capable to obtain an accurate replica suitable for SEM analysis. This procedure is applied on a Neolithic carnelian bead (hard stone) from Mali (G. Calegari Private Collection, Milan) as part of a scientific research aimed at documenting the technological indicators present on the object and their interpretation. Impressions were taken on the hole on the two faces (hole A and B) and part of the surfaces (surface A and B).Thanks to the very short contact times and the good mechanical characteristics of the used product (HD negative impression silicone) the procedure is a safe, easy, cheap, portable, low-time consuming and very effective, especially in the case of hidden surfaces such as inside of perforations.</div><style>
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[STEPS]
?. [Preliminary evaluation]
Evaluation phase
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							Careful evaluation of the degree of porosity and fragility of the object involved in the mold-making process. In our case it is a compact object, not cracked, made on a hard raw material (carnelian). It is very important to asses whether the surface may be stained by the use of low-viscosity precision dental impression silicones (by reading the scientific literature; carrying out preliminary tests on a small point of the object).
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?. [Preliminary cleaning and blocking]
Preliminary cleaning phase of the archaeological object
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							Gestures: actions prior to taking the actual mold impression. Not all actions are always necessary: for example, the last one (1.3) is mandatory in cases where we have to make impressions on an incomplete and/or unstable object.
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?. [Preliminary cleaning and blocking]
Washing. Holding the piece in your hands, you clean it with demineralised water and then with acetone using a soft, clean toothbrush or cotton (about 3').
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?. [Preliminary cleaning and blocking]
Drying. Leave the piece to dry completely in an open resealable envelope to prevent the dust from settling again (time taken about 30', at a temperature of about 18° C).
?. [Preliminary cleaning and blocking]
Creation of support and object locking. In our case (broken bead with strongly biconical holes) it is necessary to create an adhesive paste support that blocks the piece and contains the silicone. A release agent layer is not applied because it will be on contact with a hard stone.
?. [mould-making ]
Pouring phase I
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							Working time: 1’
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							Taking the molds by injection of impression material into the bead drill A and on surface of the object (3 set of drill hole impressions).Gestures: pouring by dispenser gun; ejection of a little ammount of mixed fluid (and discard it) from the dispenser to let air out of the tip; completely fill the end of the tip so that the inlet hole is full to prevent an air column from being injected (time taken: 30'').
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?. [mould-making ]
Drill hole impression A. In this case, strongly biconical holes, a impression is produced in two stages: first one face and then the other. The pouring starts from the bottom and continues with a circular movement to make the silicone adhere to the walls in the best possible way, continuing to rise in a regular way and remaining in an axial position to fill the voids.
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?. [mould-making ]
Surface impression A. Spread the silicone by injecting and advancing regularly to prevent air bubbles from forming. Leave the excesses of impression material to facilitate the subsequent demoulding phase.
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?. [mould-making ]
Hardening phase I
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							The hardening (demoulding time of silicone : 2’) of the mixture begins as soon as you start injecting the mixture.
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?. [mould-making ]
Demoulding phase I
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							Gestures: To be carried out by hand or without sharp tools (plastic tweezers) according to a continuous and delicate movement.
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?. [mould-making ]
Pouring phase II 
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							Working time: 1’
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							Taking the molds by injection of impression material into the bead drill B and on surface of the object (3 set of drill hole impressions). Gestures: after changing the mixer tip and its terminal (oral tip), everything as phase 3, step 3.1 and 3.2 (time taken: 30'').
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?. [mould-making ]
Drill hole impression B. Like step 3.1.
?. [mould-making ]
Surface impression B. Like step 3.2.
?. [mould-making ]
Hardening phase II
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							Demoulding time of silicone : 2’ 
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?. [mould-making ]
Demoulding phase II
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							Gestures: like in phase 5
?. [Examination phase and conservation of impressions]
Observation of molds by microscope (checking for imperfections, air bubbles, etc.).
?. [Examination phase and conservation of impressions]
Storage of molds in resealable plastic bags at room temperature.
?. [Final cleaning ]
Final cleaning stage of the archaeological objectRemoval of any residues of silicone impression material. Any slight halo due to the silicone oil can be removed by washing the piece with distilled water and acetone.
?. [Final cleaning ]
Washing. Use of paintbrush, distilled water and dishwashing surfactant.
?. [Final cleaning ]
Rinsing. Use of paintbrush and demineralised water.
?. [Final cleaning ]
Drying. Leave the piece to air dry completely (time taken about 15’, at a temperature of about 18° C).