Addition of glycerol to lactating cow diets stimulates dry matter intake and milk protein yield to a greater extent than addition of corn grain

The objective of this study was to determine if the addition of glycerol to the diet of dairy cows would stimulate milk protein yield in the same manner as the addition of corn grain. Twelve multiparous lactating dairy cows at 81 ± 5 d in milk were subjected to 3 dietary treatments in a replicated 3 × 3 Latin square design for 28-d periods. The diets were a 70% forage diet considered the basal diet, the basal diet with 19% ground and high-moisture corn replacing forages, and the basal diet with 15% refined glycerol and 4% added protein supplements to be isocaloric and isonitrogenous with the corn diet. Cows were milked twice a day and samples were collected on the last 7 d of each period for compositional analysis. Within each period, blood samples were collected on d 26 and 27, and mammary tissue was collected by biopsy on d 28 for Western blot analysis. Dry matter intake increased from 23.7 kg/d on the basal diet to 25.8 kg/d on the corn diet and 27.2 kg/d on the glycerol diet. Dry matter intake tended to be higher with glycerol than corn. Milk production increased from 39.2 kg/d on the basal diet to 43.8 kg/d on the corn diet and 44.2 kg/d on the glycerol diet. However, milk yield did not differ between corn and glycerol diets. Milk lactose yields were higher on the corn and glycerol diets than the basal diet. Milk fat yield significantly decreased on the glycerol diet compared with the basal diet and tended to decrease in comparison with the corn diet. Mean milk fat globule size was reduced by glycerol feeding. Milk protein yield increased 197 g/d with addition of corn to the basal diet and 263 g/d with addition of glycerol, and the glycerol effect was larger than the corn effect. The dietary treatments had no effects on plasma glucose concentration, but plasma acetate levels decreased 27% on the glycerol diet. Amino acid concentrations were not affected by dietary treatments, except for branched-chain amino acids, which decreased 22% on the glycerol diet compared with the corn diet. The decreases in plasma acetate and branched-chain amino acid concentrations with glycerol and the larger effects of glycerol than corn on milk protein and fat yields suggest that glycerol is more glucogenic for cows than corn grain.

Modeling the investment casting of a titanium crown

OBJECTIVE

The objective of this study was to apply computational modeling tools to assist in the design of titanium dental castings. The tools developed should incorporate state-of-the-art micromodels to predict the depth to which the mechanical properties of the crown are affected by contamination from the mold. The model should also be validated by comparison of macro- and micro-defects found in a typical investment cast titanium tooth crown.

METHODS

Crowns were hand-waxed and investment cast in commercial purity grade 1 (CP-1) titanium by a commercial dental laboratory. The castings were analyzed using X-ray microtomography (XMT). Following sectioning, analysis continued with optical and scanning electron microscopy, and microhardness testing. An in-house cellular-automata solidification and finite-difference diffusion program was coupled with a commercial casting program to model the investment casting process. A three-dimensional (3D) digital image generated by X-ray tomography was used to generate an accurate geometric representation of a molar crown casting. Previously reported work was significantly expanded upon by including transport of dissolved oxygen and impurity sources upon the arbitrarily shaped surface of the crown, and improved coupling of micro- and macro-scale simulations.

RESULTS

Macroscale modeling was found to be sufficient to accurately predict the location of the large internal porosity. These are shrinkage pores located in the thick sections of the cusp. The model was used to determine the influence of sprue design on the size and location of these pores. Combining microscale with macroscale modeling allowed the microstructure and depth of contamination to be predicted qualitatively. This combined model predicted a surprising result--the dissolution of silicon from the mold into the molten titanium is sufficient to depress the freezing point of the liquid metal such that the crown solidifies the subsurface. Solidification then progresses inwards and back out to the surface through the silicon-enriched near-surface layer. The microstructure and compositional analysis of the near-surface region are consistent with this prediction.

SIGNIFICANCE

A multiscale model was developed and validated, which can be used to design CP-Ti dental castings to minimize both macro- and micro-defects, including shrinkage porosity, grain size and the extent of surface contamination due to reaction with the mold material. The model predicted the surprising result that the extent of Si contamination from the mold was sufficient to suppress the liquidus temperature to the extent that the surface (to a depth of approximately 100 microm) of the casting solidifies after the bulk. This significantly increases the oxygen pickup, thereby increasing the depth of formation of alpha casing. The trend towards mold materials with reduced Si in order to produce easier-to-finish titanium castings is a correct approach.

Modeling the surface contamination of dental titanium investment castings

OBJECTIVE

The objective of this study was to develop a computational tool for assisting the design of titanium dental castings with minimal defects and to compare computational simulations with casting experiments.

METHODS

Modeling. An in-house cellular-automata solidification and finite-difference diffusion program was coupled with a commercial casting program and applied to (a) simple geometric wedge models and (b) a 3D-laser scan of a molar crown casting. Experimental. Wedges and molar crowns were hand-waxed and investment cast in commercial purity grade 1 (CP-1) titanium by a commercial dental laboratory. The castings were sectioned and analyzed using light and scanning electron microscopy, X-ray microanalysis, and microhardness testing.

RESULTS

In the wedge sample, contamination with impurities (Al, Si), including intermetallic precipitates, was found to extend to a depth ranging from 30 to 120 microm depending on the section thickness and hence the local cooling rate. Microstructural and mechanical (hardness) effects were found to a depth ranging from 80 to 250 microm. The coupled micro/macro model predictions showed reasonable agreement for the pattern of contamination.

SIGNIFICANCE

Dental and medical applications demand close dimensional tolerance and freedom from surface impurities and structural flaws in castings having unique shapes. The ability to predict the structural, mechanical, and chemical changes resulting from the casting process will help to design the casting and post-casting processes to minimize these problems.

An evaluation of slot size in orthodontic brackets--are standards as expected?

The slots of five upper left central incisor brackets from 11 commercially available bracket systems (3M Unitek, Monrovia, Calif: Twin Torque Roth, Clarity MBT, and Victory Series MBT; Dentarum, Pforzheim, Germany: Discovery Roth (0.56 mm) and Elegance Plastic Roth; Forestadent, Pforzheim, Germany: Mini Mono MBT; TP LaPorte, Indiana: Nu-Edge Roth and Mxi Advant-Edge Roth; Ormco Corp., Orange, Calif: Damon II SL Roth; Ortho Organizers, San Marcos, Calif: Elite Mini Opti-MIM Roth and Elite Mini Opti-MIM MBT) were measured in the 0.022-inch (0.5588 mm) dimension. Measurements were taken after operator calibration, and a digital readout was produced. Results indicate that all bracket slots are oversized. Three bracket systems slots (Twin Torque, Clarity, and Mini Mono) were within 5% (+/-1.08, 1.655, 1.75) of their stated dimensions with essentially parallel slot walls. The Elegance Plastic slot was parallel sided but oversized by 12% (+/-1.15). The geometry of bracket slots was also variable. The Victory Series slot was slightly divergent with the top oversized by 6% (+/-1.035). The Nu-Edge slot was divergent and slot top oversized by 14% (+/-1.32). The Mxi Advant-Edge, Damon II SL, Elite Mini Opti-MIM Roth, and MBT were all convergent, and the base of the Damon slot was oversized by 17% (+/-1.79). The Discovery bracket was convergent, and the slot base was oversized by 24% (+/-1.255), which was the largest recorded variance. This bracket also had a 7% difference between the widths of the slot top and the base. Inaccurate machining of bracket slot dimensions and the use of undersized archwires may directly and adversely affect three-dimensional tooth positioning.

The longitudinal accuracy of fit of titanium implant superstructures superplastically formed on investment models

OBJECTIVES

To measure the effect of three special liquid proportions and five forming temperatures on the length of superplastically formed titanium dental implant superstructures made using Rematitan dental casting investment.

METHODS

Rematitan dental casting investment was used to make models that contained two implant abutment analogues 8 mm apart. The investment was hand-mixed with 30, 50 or 70% special liquid proportions and ten models of each were made. Dies for superplastic forming (SPF) were made that contained three models (one for each special liquid proportion). Five dies were made for forming at 810 degrees C and five dies for forming at 810, 840, 873, 900 and 927 degrees C using 2 mm thick Ti-6Al-4V discs. The distance between the abutment impressions in the formed discs was measured with a traveling microscope and compared with the distance between the abutments of the master model to determine the most accurately fitting superstructure beam.

RESULTS

The proportion of special liquid used to make the SPF models had an unpredictable effect on the length of the superstructure at each forming temperature. Superstructure beams formed at lower temperatures had a better fit compared to the master model for all special liquid proportions. The most accurately sized beams were formed at 810 degrees C.

SIGNIFICANCE

SPF can provide an alternative method for the production of dental implant superstructures. The fit was affected predominantly by the forming temperature, but also by the special liquid proportion used for making the SPF investment model.

Numerical simulation of the superplastic forming of dental and medical prostheses

A relatively recent development in prostheses manufacture is the use of superplastic forming technology formerly associated with industrial applications such as aerospace components. This paper considers the finite element incremental flow formulation for the simulation of the forming of thin and moderately thick titanium alloy sheet dental and medical prostheses and, in particular, the determination of the pressure-time cycle. Issues concerning material characteristics are discussed, and the process is illustrated by a number of examples. The application of finite element simulation in this context is ongoing, nevertheless interim conclusions are drawn and future directions for research are identified.

Measurement of the setting and thermal expansion of dental investments used for the superplastic forming of dental implant superstructures

OBJECTIVES

The study examined the setting and thermal expansions of seven dental casting investments intended for use in the superplastic forming (SPF) of Ti6A14V for dental implant superstructures. The total expansion is compared with the coefficient of thermal expansion of the alloy.

METHODS

The unrestricted setting expansion of the investment was measured at room temperature using a lined brass trough and dial gauge. The thermal expansion was measured at a heating rate of 5 degrees C/min up to 900 degrees C. The thermal expansion of Ti6A14V was measured at a heating rate of 15 degrees C/min.

RESULTS

The Selevest investment had a small shrinkage on setting. The largest setting expansions were observed with Rema Exakt and Fujivest. Selevest DM and Fujivest had the greatest thermal expansions. Rema Exakt had the greatest overall expansion. The total expansion of Rematitan most closely matched the thermal expansion of Ti6A14V when the manufacturer's recommended special liquid to water ratio for crowns and bridges was used.

SIGNIFICANCE

SPF is a novel technique which is being applied to the manufacture of dental prostheses. As dental casting investments are used to make the dies used for SPF the dimensional changes associated with these materials should be matched to the dimensional changes of the Ti6A14V alloy used in order to achieve a passive fit. This study has identified Rematitan as an investment whose total expansion may enable an accurately fitting SPF implant superstructure to be made.

Dentine chips produced by nickel-titanium rotary instruments

This study aimed to compare the cross-sectional shape of two nickel-titanium rotary instruments, namely ProFile and Quantec files, both ISO 25, 0.06 taper, and sought to relate this to the chips produced by cutting dentine. A limited comparison was made with stainless steel engine reamers. First, five files of each type were sectioned transversely at 12 mm, 8 mm and 4 mm from the tip and examined by scanning electron microscopy. The cutting angles were assessed by a direct measurement technique which allowed for the inclination of a cutting edge to the root canal. Second, eight samples of cutting debris were collected from instrumentation by each type of nickel-titanium file and four samples from the engine reamers. The major and minor axis, area and roundness of the dentine chips in each sample were measured using computerized particle analysis. The results demonstrated that all files had a negative cutting angle which varied at the different levels (ProFiles range 69.4 degrees to 58.4 degrees and Quantec range 74.8 degrees to 56.8 degrees). The consistency within files of the same type was good as demonstrated by low standard deviations, except for Quantec files at the 4 mm level where higher standard deviations of 4.1 degrees and 5.5 degrees for the two blades were found. The chip analysis showed significant differences between chips produced by ProFile and Quantec files (P < 0.05). The latter were larger and rounder. The chips from the ProFile and the engine reamer chips were similar in dimension (P > 0.05). No simple relationship existed between file geometry and the dentine chips produced during instrumentation.

A comparison of two nickel-titanium instrumentation techniques in teeth using microcomputed tomography

AIM

The aim of the study was to compare the shaping of root canals by two nickel-titanium instrumentation techniques using microcomputed tomography (MCT).

METHODOLOGY

Ten mandibular first molar teeth (30 canals) that had intact crowns and fully formed roots were scanned using MCT. Fifteen canals were instrumented using NiTiFlex hand files (Maillefer) using balanced force. The remainder were instrumented using prototype ProFile 0.04 Taper instruments (Dentsply) in a crown-down manner to an apical size ISO 25. The teeth were scanned again following instrumentation. The two instrumentation techniques were compared in a total of 27 canals. The area of dentine removed at predetermined levels (2.0, 3.0, 4.5, 6.0, 7.5 mm) from the apex was measured. Transportation and centring were recorded. Images constructed at these levels were compared with video images of equivalent physical sections created after the second scan. The volume of dentine removed in the apical 7.5 mm of the root canals of each tooth was calculated and the different techniques compared. Rendered three-dimensional images were used to assess the preparations qualitatively. The time taken for preparation was recorded.

RESULTS

There was no significant difference between hand instrumentation with NitiFlex files and machine instrumentation with prototype ProFile 0.04 Taper instruments for any of the variables tested.

CONCLUSIONS

Both techniques produced well centred and tapered preparations.

The influence of handling technique on the strength of phosphate-bonded investments

OBJECTIVES

This study examined the influence of six handling techniques on the strength of four phosphate-bonded investment (PBI) materials. The size and distribution of pores produced by the handling techniques were investigated and their effect on room temperature strength determined.

METHODS

Specimens were tested for strength using four point bend at a cross-head speed of 1 mm min-1. The fracture surface of randomly selected specimens was viewed through a light microscope to determine the pore size. A three-parameter Weibull distribution was used to describe strength giving values of Weibull modulus and characteristic strength.

RESULTS

The handling technique that produced the smallest pore size resulted in the highest strength. Significant differences in strength (90% confidence level) were found between most handling techniques. The reduction of pore size by mechanical spatulation and setting under pressure was found to be the most effective method of increasing the strength of PBI.

SIGNIFICANCE

Previous studies have related handling technique to the inclusion of air bubbles in dental investments but have not reported any consequent effect on strength. PBI materials are used in production of a range of dental appliances with a variety of different material forming processes and are required to withstand the loads applied without fracture or distortion to ensure accuracy of fit of these prostheses. The use of a handling technique that reduces pore size can significantly increase the strength of PBIs, thereby decreasing the possibility of die fracture or degradation.

Dental materials: 1997 literature review

This review of the published literature on dental materials for the year 1997 has been compiled by the Dental Materials Panel of UK. It continues a series of annual reviews started in 1973. Emphasis has been placed upon publications, which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, ceramometallic restorations and resin-bonded bridges, ceramics, denture base resins and soft lining materials, impression materials, dental implant materials, orthodontic materials, biomechanics and image processing, resin composites, and casting investment materials and waxes). Three hundred and thirty three articles have been reviewed.

Micro-computed tomography: a new tool for experimental endodontology

AIM

Micro-computed tomography (MCT) using conebeam geometry is a method of producing true 3D images of the structure of small samples. A prototype MCT unit was adapted for imaging teeth to examine whether it could be used to quantify the instrumentation of root canals.

METHODOLOGY

Ten mandibular first molar teeth that had intact crowns and fully formed roots were scanned using MCT at a resolution of 0.081 mm and 3D-rendered images created; root canals were segmented from this. Reproducibility of MCT was verified for root canal shape and size. Access cavities were prepared into the pulp space and root canals enlarged to a continously tapering preparation using a crowndown technique. Each tooth was scanned again to allow comparison of pre- and post-instrumentation images. The roots were then sectioned at five predetermined horizontal levels for video-digitized measurement of dimensions of roots and root canals. The video images had a resolution of 0.025 mm. Video-digitized images of the physical cut surfaces were compared with equivalent MCT reconstructed images. The total area of the root canals (internal) and root (external) at each level were calculated from both MCT reconstructions and video-digitized images, and compared.

RESULTS

There was a highly significant correlation between MCT and video images for both external and internal areas (r = 0.94). Rendered 3D images were constructed to show the root canal systems of teeth. The total volumes of the apical 7.5 mm of root canals were calculated from rendered images of nine teeth before and after instrumentation. The mean amount of dentine removed by instrumentation was 3.725 mm3, which was 28% of the original canal volume.

CONCLUSIONS

Micro-computed tomography was shown to be accurate for experimental endodontology.

Dental materials: 1996 literature review. Part 2

This critical review of the published literature on dental materials for the year 1996 has been compiled by the Dental Materials Panel of the UK. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, resin-bonded bridges and ceramo-metallic restorations, ceramics, denture base resins and soft lining materials, impression materials, implants materials, orthodontic materials, biomechanics and image processing, resin composites and casting investment materials and waxes). Three hundred and thirteen articles have been reviewed.

Dental materials: 1996 literature review

This critical review of the published literature on dental materials for the year 1996 has been compiled by the Dental Materials Panel of the UK. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, resin-bonded bridges and ceramo-metallic restorations, ceramics, denture base resins and soft lining materials, impression materials, implants materials, orthodontic materials, biomechanics and image processing, resin composites and casting investment materials and waxes). Three hundred and thirteen articles have been reviewed.

Stress-strain and thermal expansion characteristics of a phosphate-bonded investment mould material for dental super plastic forming

OBJECTIVES

The stress-strain and thermal expansion characteristics of a phosphate-bonded investment material were measured to determine its suitability as a die material for superplastic forming of dental appliances. Titanium alloy denture bases and implant superstructures have been fabricated successfully using this investment material, but a greater understanding of its properties was sought to optimize the forming technique before exploitation could proceed. The effect of a boron nitride powder on the properties of phosphate-bonded investment was also investigated.

METHODS

Three-point bend test specimens were cast in the phosphate-bonded investment material. Four groups of test samples were produced at specified water-to-powder ratios (w/p ratios). For one group 5 wt.% boron nitride powder was added to the investment powder before mixing with water. For stress-strain measurements specimens were heated to 920 degrees C and held for 30 min prior to the application of load. For thermal expansion measurements specimen length was measured for the entire heating and cooling cycle.

RESULTS

Curves of stress versus strain and thermal expansion were recorded for all four test groups. For increasing initial w/p ratio failure stress decreased with lowest failure stresses for specimens containing boron nitride. Strain to failure also decreased with increasing initial w/p ratio, except for the group containing boron nitride which showed relatively large elongations to fracture. Specimen surfaces with higher initial w/p ratios were observed to have larger surface porosity. Thermal expansion curves for three groups were very similar, but differed considerably from the group containing boron nitride.

CONCLUSIONS

The lowest initial w/p ratio should be used for highest hot strength. Hot deformation of phosphate-bonded investment dies for superplastic forming is an issue that requires further investigation because of potential die deformation and the resulting loss of fit between the superplastic superstructure and implant components. Boron nitride could be used in phosphate-bonded investment to decrease thermal expansion, but the consequences of reduced hot strength and increased strain to failure may make the modified material unsuitable as a die for superplastic forming.

Cutting pattern of nickel-titanium files using two preparation techniques

The aim of this study was to examine the cutting patterns of recently introduced nickel-titanium endodontic files and compare them with similarly shaped stainless steel files using both balanced-force and stepback techniques. Simulated canals were constructed in resin blocks with 40 degrees curvature. Fifty blocks were prepared by a balanced-force technique: 25 with Nitiflex (Maillefer) and 25 with Flexofiles (Maillefer) to master apical file size 40. Fifty blocks were prepared by a stepback technique: 25 with Nitiflex and 25 with Flexofiles to a master apical file size 30. Composite prints were made of pre- and post-instrumentation canals using a computerized imaging system. The amount of material removed from the inner and outer canal curvatures was measured at eight levels in the apical 11 mm of the canal. The results were analysed by Kruskal-Wallis ANOVA. Using the balanced-force technique, Nitiflex files removed less material on the outer curve apically and less on the inner curve mid-canal (P < 0.05), while Flexofiles caused more apical transportation. In the stepback technique, the Nitiflex files removed more material from the outer curve apically (P < 0.05); the Flexofiles removed more material along the entire inner curve (P < 0.05), and maintained canal curvature better than Nitiflex files. It appeared preferable to use Nitiflex files in a balanced-force technique, and Flexofiles in a filing technique as stainless steel files can be precurved.

Cutting pattern of Flexogate instruments in plastic blocks

Design modifications of endodontic instruments have been made to improve instrumentation in curved root canals. A new instrument, the Flexogate (Maillefer), has been introduced to overcome some of the problems associated with preparation of the apical part of the root canal. The purpose of this study was to investigate the cutting pattern of Flexogates in curved canals (45 degrees) in clear plastic blocks, prepared by four different operators. Image analysis was used to take accurate measurements of canal shape before and after instrumentation to calculate cutting patterns. The results showed that Flexogates did not remain centred in the canal of a plastic block during cutting; maximum transportation occurred in the apical third of the outer curve, with little or none in the middle and coronal thirds. The clinical implications of this study require further investigation of the effect of coronal preflaring, and experimentation in teeth.

An investigation of debonding between heat-cured PMMA and titanium alloy (Ti-6A1-4V)

OBJECTIVES

The bonding of acrylic resin to dental prostheses constructed from metal has been the subject of extensive research in recent years, much of it between base-metal alloys such as Ni-Cr alloys and dental composite. The development of techniques for the manufacture of dental appliances from titanium and titanium alloys has opened new avenues for investigation of the metal/acrylic resin bond.

METHODS

In this study, a bond strength comparison between two PMMA products, Trevalon and Metadent, and discs of titanium alloy, Ti-6A1-4V, was investigated using a four-point bend test configuration. Trevalon is a standard, commercially available, heat-cured acrylic resin, while Metadent is an acrylic resin incorporating a chemical bonding agent, 4-META. A comparison was made between two processes that are routinely used to enhance the bond between metal and acrylic, namely sandblasting of the metal surface and using the Silicoater system following sandblasting. Each of the samples was paired, thus allowing a further comparison to be made between samples that had undergone thermocycling in a water bath, with those that had been held at a constant temperature.

RESULTS

The study revealed that a superior metal/acrylic bond was achieved by the use of the silicoating system when either Trevalon or Metadent was used both before and after thermocycling. Reduced debonding loads were recorded for all samples that had undergone thermocycling.

CONCLUSIONS

Highest bond strengths were recorded for Trevalon or Metadent processed against a silicoated Ti-alloy surface.

Dental materials: 1995 literature review

This critical review of the published literature on dental materials for the year 1995 has been compiled by the Dental Materials Panel of the United Kingdom. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, resin composites, dentine bonding, dental amalgam, endodontic materials, casting alloys, investment materials, resin-bonded bridges and ceramo-metallic restorations, all ceramic restorations, denture base and soft lining materials, impression materials, dental implants, orthodontic materials and biomechanics). Three hundred and thirty articles published in 68 titles have been reviewed.

Alternatives to gold

Despite, or perhaps because of, its antiquity the lost wax casting process which we use in dentistry still produces the most accurate and durable restorations in our armamentarium. Although high-gold content alloys still act as the benchmark of quality, fit, and durability in the aggressive conditions of the mouth, their inherent cost, high density and low modulus have stimulated a search for alternatives. This has ranged widely through the metallurgical world and across the periodic table of the elements. As a result dental alloys for casting into inlays, crowns and bridges (with or without facings of dental porcelain) have become incredibly complex. This article will explain some of the reasons behind the developments.

The use of dental amalgam--an art or a science?

Dental amalgam remains the best material for filling teeth when aesthetics are of secondary importance. Over the last 20 years improvements in the composition have reduced marginal failure due to creep and corrosion, and the use of modern adhesives has lead to an early seal between the tooth and the filling. However, the proper handling by the dentists remains as the cornerstone for fillings that are going to last. This article describes the history, classification, structure and handling variables of dental amalgams. It also discusses the effects of extra elements, bonding agents and the possible hazard of mercury.