Femtosecond laser processing for blast-hole analysis: laser removal of slurry and effect on rocks

Andrei V. Rode; Andrew J. Hill; Ehsan Mihankhah; John Zigman; Julia Brand; Ksenia Maximova; Ludovic Rapp; Steve Madden

REVIEW 3 major objections 5 minor 2 references

Femtosecond laser processing for blast-hole analysis: laser removal of slurry and effect on rocks

T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read Femtosecond laser can strip blast-hole slurry up to 1.5 cm thick and leave rock that FTIR and VIS/NIR spectroscopy can still identify.

desk verdict Useful lab data on femtosecond laser slurry removal for blast-hole analysis, but the alteration depth is asserted, not measured, and that gap sits exactly under the paper's central claim. read the letter →

arxiv 2412.06214 v1 pith:VDIDC2PM submitted 2024-12-09 physics.app-ph

Julia Brand , Ksenia Maximova , Steve Madden , Andrei V. Rode , Ehsan Mihankhah , John Zigman , Andrew J. Hill , Ludovic Rapp This is my paper · ORCID

classification physics.app-ph

keywords femtosecondlaserablationblast-holeslurryironoregoethiteFTIRspectroscopyVIS/NIRcleaningmininggeology

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper argues that a 1030 nm femtosecond laser can remove iron-ore slurry from blast-hole walls quickly and gently enough that the underlying rock remains identifiable by standard field spectroscopy. It reports slurry ablation thresholds near 0.35 $J/cm^2$ regardless of water content, rock ablation thresholds near 0.30–0.40 $J/cm^2$, and efficient removal at fluences around 4–6 $J/cm^2$. In the demonstrator, about 1.5 cm of slurry was cleared from goethite, and FTIR plus VIS/NIR spectra of the exposed stone still matched goethite reference features despite surface darkening. If the result transfers to real holes, it gives mine geologists a contactless way to open analysis windows without destabilising the wall.

What carries the argument

The carrying mechanism is femtosecond (275 fs) infrared (1030 nm) laser ablation in the single-shot-per-spot regime: a polygon-galvo scanner moves the beam so fast that each location receives one pulse, avoiding heat accumulation. The key measured quantities are depth per pulse and volumetric ablation efficiency as functions of fluence, which define a processing window around 4.0–6.0 $J/cm^2$ for efficient slurry removal and expose the lower discolouration thresholds of the rocks. Optical profilometry, colourimetry, FTIR, and VIS/NIR spectroscopy serve as the read-out that the underlying rock remains identifiable, while the Rayleigh range of the focused beam sets the practical constraint on how thick a slurry layer can be removed before refocusing is needed.

What would settle it

Take a slurry-covered wall segment from an actual blast hole with field moisture above 14 wt% and clasts up to 5 cm, ablate a window at 5.4 $J/cm^2$ under the same 1030 nm, 275 fs conditions, and check whether FTIR and VIS/NIR spectra still identify the underlying rock and whether the discoloured layer is truly only a few micrometres deep; either failure would weaken the central claim.

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Extended reading notes

Core claim

The central claim is that femtosecond laser ablation is a viable contactless method for opening analysis windows through blast-hole slurry while preserving the diagnostic spectral signature of the rock underneath. The authors establish ablation thresholds and measure depth-per-pulse and efficiency curves for slurry and for five economically relevant rocks, then show that laser exposure darkens the rock surface but does not erase the absorption bands used for mineral identification. The strongest demonstration is goethite covered by about 1.5 cm of slurry at 3 wt% water: after 100 scans at 5.4 $J/cm^2$ the slurry was gone and FTIR and VIS/NIR spectra still showed goethite's Fe–O, OH, and Fe$^{3+}$ electronic features, with only a reduced OH band and a changed Fe$^{3+}$ slope. They conclude that advanced spectroscopy can identify the rock despite the colour change, and suggest mid-IR wavelengths as a future route to reduce discolouration.

Load-bearing premise

The demonstration rests on laboratory slurry made by re-wetting dried cone material at water contents below 14 wt%; if real blast-hole slurry, which is wetter, thicker, full of centimetre-scale clasts, and found on vertical walls, ablates differently, the reported cleaning window may not transfer.

Editorial extensions

If this is right

A femtosecond laser can cut analysis windows through slurry layers at least 1.5 cm thick, and thicker layers are reachable by refocusing the beam to stay within the Rayleigh range.
Dry slurry (under 1 wt% water) removes about two orders of magnitude faster than wet slurry, reaching 482 $mm^3/min/W$ at 5.3 $J/cm^2$, so drying state strongly changes throughput.
For hematite, goethite, banded iron, limonite, and shale, fluences near 4–6 $J/cm^2$ (up to about 7.7 $J/cm^2$ for limonite) balance removal rate against discolouration while keeping the rocks' diagnostic infrared bands intact.
Because slurry and rock share composition, complete removal in the demonstration required over-ablating a small thickness of the stone once the slurry was gone, leaving a thin laser-altered skin on the exposed window.
The discolouration of iron-bearing rocks at 1030 nm is linked to an Fe$^{3+}$ electronic transition near 0.6 µm, so moving to mid-infrared wavelengths should reduce the visible darkening in future systems.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The laboratory slurry was made by re-wetting dried cone material and tested below 14 wt% water, so a field trial on naturally wet, clast-rich slurry on vertical walls is the natural next test before the method can be called operational.
The paper's statement that discolouration extends only a few micrometres is inferred rather than measured; a cross-section of an irradiated rock would settle whether the altered layer is thin enough for bulk spectroscopy to ignore.
The same single-shot-per-spot scanning logic should extend to other mining and exploration coatings, such as dust crusts or cemented fines, wherever a contactless window through an obscuring layer is needed.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The genuinely new thing is the ablation dataset: thresholds, rates, and efficiencies for iron-ore blast-hole slurry at several water contents and for five economically relevant Australian rocks (banded iron, limonite, goethite, shale, hematite) at 1030 nm, 275 fs. I know of no other paper that gives these numbers. The methodology is thorough and the numbers look internally consistent. The authors also did something worth credit: they built a 1.5 cm thick slurry layer, removed it with a scanning fs laser, and showed that the exposed goethite is still spectroscopically recognisable. That is a clean feasibility demonstration.

The soft spots, in order of importance.

First, the paper asserts the laser-induced alteration is 'a few µm' deep without measuring it. Section 3.b states it; the conclusion repeats it. This is load-bearing. The same sections report fluence-dependent decreases in the 3400 cm-1 OH band and a change in the Fe3+ transition slope around 0.6 µm. If the altered layer is thicker than the sampling depth of DRIFT or VIS/NIR, then the spectra the authors compare are partly spectra of the altered material, and the identification claim weakens. The authors themselves note that DRIFT may 'go through the damage layer instead of measuring it.' A cross-section or depth-resolved measurement would have settled this. Without it, the central claim is under-supported, not wrong.

Second, the transfer to real blast holes is not demonstrated. The slurry was re-wetted from dried cone material, tested at water contents up to 14 wt%, with the key demonstration at 3 wt%. Real slurry is thicker, wetter, contains clasts up to 5 cm, and sits on vertical walls. The authors correctly call this a feasibility study, but they still conclude that the method offers 'significant advantages' for mine planning. That conclusion outruns the evidence.

Third, the goethite identification is qualitative—'clear when compared to a reference'—with no quantitative similarity measure. Fine for a first look, but the claim that FTIR and VIS/NIR 'can still accurately identify the rock' needs a classifier or at least a metric.

Minor notes: data availability is 'on request' only; a public dataset would help. The self-citations are heavy but to genuinely relevant earlier work on fs laser cleaning of heritage stone; I don't see inflation.

Overall: this is a solid experimental characterisation paper with useful, reproducible-looking data, and a clear industrial motivation. It needs revision to measure alteration depth and to tone down the field-readiness claims. I would send it to peer review; a good referee can force the missing depth measurement or an explicit statement that the identification claim applies only under the tested lab conditions.

Referee Report

3 major / 5 minor

Summary. This manuscript reports an experimental study of femtosecond laser processing (275 fs, 1030 nm) for removing iron ore slurry from blast holes and exposing the underlying rock for geological analysis. The authors measure ablation thresholds, depths per pulse, and ablation efficiencies for slurry at several water contents and for five rock types (banded iron, limonite, goethite, shale, hematite). They characterize laser-induced surface changes using optical microscopy, profilometry, colourimetry, FTIR, and VIS/NIR spectroscopy. They demonstrate removal of a 1.5 cm thick slurry layer from a goethite block at 3 wt% water content using 5.4 J/cm2, and they conclude that FTIR and VIS/NIR spectroscopy can still accurately identify the rock despite surface discolouration. The paper is a characterization study with detailed experimental procedures and internally consistent numbers, but the central claim about preserving spectroscopic identifiability rests on assertions about the alteration depth and on a qualitative single-sample demonstration.

Significance. If the central claim holds, the work provides a contactless method for removing blast-hole slurry that could enable automated or robotic mine-face analysis without disrupting hole-wall stability, which is relevant for mine planning and grade control. The paper's strengths are the systematic measurement of ablation thresholds and efficiencies for realistic industrial materials, the explicit comparison of slurry water contents, and the use of multiple complementary characterization techniques. However, the key conclusion that spectroscopy can still accurately identify the rock is currently supported only by qualitative evidence and by an unmeasured assertion that laser alteration is confined to a few micrometres. The observed changes in colourimetry, OH band intensity, and Fe3+ transition slope show that alteration is non-negligible, so the validity of the central claim is not yet firmly established.

major comments (3)

[core] The claim that laser-induced discolouration 'appeared shallow and occurred only at a depth of a few µm' is asserted without supporting measurement. No cross-section, depth-resolved spectroscopy, or stepwise removal experiment is presented. This is load-bearing for the central conclusion that FTIR and VIS/NIR can still identify the rock, because DRIFT and reflectance measurements integrate signal over a depth comparable to or larger than the presumed alteration layer. If the altered layer is thicker than the information depth of these techniques, the spectra would predominantly reflect altered material. The manuscript itself notes in §3.c.ii that 'DRIFT may go through the damage layer instead of measuring it.' Please provide a direct measurement of the alteration depth (e.g., SEM/EDS cross-section, confocal Raman depth profiling, or controlled etching/re-measurement) and use it to bound the validity of the spectroscopic identification claim.
[core] The demonstration of goethite identification after slurry removal is qualitative and based on a single sample: the text states that identification was 'clear when compared to the reference spectrum' without any quantitative similarity metric, spectral library classifier, or error analysis. This matters because the same sample shows substantial measured changes—colourimetry at 1.5 J/cm2 gives an 82% decrease in a* and 86% decrease in b* for goethite, and the FTIR OH band at 3400 cm-1 decreases with fluence—yet the demonstration was run at 5.4 J/cm2, which exceeds the fluence used for the damage characterization. To support the claim that spectroscopy can still 'accurately identify the rock,' please include a quantitative identification test (e.g., spectral angle mapper, correlation with reference spectra, or a trained classifier) on multiple samples and at the fluence used for slurry removal.
[core] The transferability from the laboratory setup to real blast-hole conditions is not demonstrated. The slurry samples were prepared by re-wetting dried cone material from six blast holes and tested at water contents between <1 wt% and 14 wt%, with the demonstration performed on a horizontal sample at 3 wt% water. Real blast-hole slurry is described in the introduction as varying from a few millimetres to several centimetres thick, containing clasts up to roughly 5 cm, and coating vertical walls. The paper does not address how ablation efficiency, pebble removal, or slurry flow would behave on vertical surfaces or at higher water contents. Please either extend the demonstration to more representative conditions or explicitly scope the conclusions to the conditions tested and discuss the engineering steps needed for field deployment.

minor comments (5)

[Title/Abstract] The word 'e9ect' in the title appears to be an OCR error for 'effect' and should be corrected.
[§4] The phrase 'as described in section 4' is self-referential; the FTIR and VIS/NIR results are described in Section 3.c, not Section 4.
[Fig. 7 caption] The caption contains an unbalanced parenthesis and the phrase 'in function of the laser fluences' should read 'as a function of the laser fluences.'
[Table 3] The text states that a* and b* 'decreased by 105%' and '-112.5%' for some rocks; since these parameters crossed zero and changed sign, a percentage decrease is ambiguous. Please report the actual coordinate changes or clarify the sign convention.
[§2.c] The water content calculation is described as the ratio of remaining water to dry slurry, but the method for determining the dry mass (drying temperature, duration) is not specified; please provide this detail for reproducibility.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the ablation thresholds, efficiencies, and spectroscopic outcomes are measured outputs, not quantities derived from the model inputs.

full rationale

This is an experimental characterization study with no derived predictive chain that reduces to its own inputs. The ablation thresholds, depth-per-pulse values, ablation efficiencies, colourimetry changes, FTIR band changes, and VIS/NIR slope changes are all directly measured outputs reported with uncertainties. The choice of 5.40 J/cm2 for the goethite demonstration is an operating point selected from the measured efficiency curves, not a fitted parameter renamed as a prediction. The claim that FTIR and VIS/NIR spectroscopy can still identify the rock is supported by the measured spectra of the ablated goethite sample compared with a goethite reference spectrum, so the identification claim is an experimental observation rather than a tautology. The authors do include a relevant limitation statement: the FTIR section notes that 'DRIFT may go through the damage layer instead of measuring it,' and the conclusion that discolouration is limited to a few micrometres is stated without a direct depth-resolved measurement. These are validity and transferability concerns, not circularity: they do not make any output equivalent to an input by construction. Self-citations to the authors' earlier femtosecond laser cleaning studies are used as background motivation for the technique, not as load-bearing evidence for the measured ablation parameters or the spectroscopic identification. Therefore the paper is self-contained against external benchmarks for its central measurements, and the circularity score is 0.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The paper has no derivation; the central feasibility claim rests on the representativeness of prepared slurry and rock samples, the validity of the single-shot-removal regime, and the assumption that surface discolouration does not compromise spectral identification. These are domain assumptions, not standard math. No invented entities. The measured ablation parameters are outputs, not inputs, but the chosen operating fluence and water content for the demonstration are experimental choices that affect generalization.

free parameters (4)

Slurry water content in demonstration = 3 wt%
The goethite demonstration used slurry dried to 3 wt% water, which is much drier than the 9-14 wt% wet slurry and far from the 'dry <1 wt%' case with 482 mm3/min/W; removal rate and applicability for wet slurry are not demonstrated.
Operating fluence for demonstration = 5.40 ± 0.05 J/cm2
Selected from measured optimum efficiency window of 4-6 J/cm2; the result could differ at other fluences.
Ablation threshold of slurry = 0.35 ± 0.05 J/cm2
Measured from depth-per-pulse curves; sets the lower bound for removal. Reported with uncertainty but without confidence intervals.
Ablation threshold of rocks = 0.30-0.40 ± 0.05 J/cm2
Measured for all five rock types; a single range is quoted despite different compositions.

assumptions (4)

domain assumption The laser operates in the single-shot-per-spot regime at scan speeds up to 880 m/s, avoiding heat accumulation effects.
Invoked in Section 2b to support the 'cold ablation' premise. If thermal accumulation occurs, the damage assessment changes.
domain assumption Re-wetted dried slurry from six blast-hole cones is representative of real in-situ blast-hole slurry.
Section 2c states slurry characterisation is beyond scope; this assumption underpins the practical claim.
domain assumption Flat cut billets represent in-situ rock surfaces for laser interaction.
Section 2c notes samples were cut into flat billets, goethite had uneven surfaces, and shale was soft; real blast-hole walls are curved, rough, and may have different morphology.
domain assumption Surface discolouration is confined to a few micrometres and does not affect spectral identification.
Section 3b states discolouration 'appeared shallow and occurred only at a depth of a few µm' but no cross-section was measured. FTIR OH band loss indicates chemical change.

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Cite this review

Pith. "Pith review of Femtosecond laser processing for blast-hole analysis: laser removal of slurry and effect on rocks." pith.science (2026). https://pith.science/paper/VDIDC2PM

@misc{pith2026241206214,
  author       = {Pith},
  title        = {Pith review of: Femtosecond laser processing for blast-hole analysis: laser removal of slurry and effect on rocks},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VDIDC2PM}},
  note         = {Machine review of arXiv:2412.06214}
}

read the original abstract

This study investigates the possibility of using a femtosecond pulse laser to remove iron ore slurry used to stabilise blast-hole structures by mining industries, intending to preserve the wall's stability and the chemical and compositional properties of the underlying rock. In situ minerals are often coated in other material deposits, such as dust or slurry in blast holes. To analyse the rock materials beneath, its surface must be exposed by removal of the surface layer. The ablation depth per pulse and ablation efficiency of the slurry were determined using femtosecond laser pulses. Then, the ablation of rocks of economic interest in Australia, including banded iron, limonite, goethite, shale, and hematite, was studied to establish their ablation thresholds and rates. Any damage induced by the laser was investigated by optical microscopy, optical profilometry, colourimetry, VIS/NIR spectroscopy and Fourier Transform Infrared spectroscopy (FTIR).

Discussion (0). Continue with ORCID to comment.

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Works this paper leans on

2 extracted references · 1 canonical work pages

[3]

Automation and Artiﬁcial Intelligence Technology in Surface Mining: A Brief Introduction to Open-Pit Operations in the Pilbara,

Results a. Ablation threshold, rate, e8iciency, and e8ects on the slurry Sections of slurry prepared with di?erent water content were ablated at varying ﬂuences. The ablation threshold was found to be 0.35 ± 0.05 J/cm2 for all tested samples. The water content did not inﬂuence the ablation threshold. Discolouration of the slurry surface occurred at ﬂuence...

arXiv 2022
[13]

Laser cleaning of stone materials: an overview of current research,

J. Brand, A. Wain, A. V . Rode, S. Madden, P . King and L. Rapp, Femtosecond pulse laser cleaning of spray paints from heritage stone surfaces, Optics Express 30 17 (2022) 31122 [14] J. S. Pozo-Antonio, A. Papanikolaou, K. Melessanaki, T. Rivas and P . Pouli, Laser-Assisted Removal of GraHiti from Granite: Advantages of the Simultaneous Use of Two Wavelen...

work page 2022

Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.

[1] [3]

Automation and Artiﬁcial Intelligence Technology in Surface Mining: A Brief Introduction to Open-Pit Operations in the Pilbara,

Results a. Ablation threshold, rate, e8iciency, and e8ects on the slurry Sections of slurry prepared with di?erent water content were ablated at varying ﬂuences. The ablation threshold was found to be 0.35 ± 0.05 J/cm2 for all tested samples. The water content did not inﬂuence the ablation threshold. Discolouration of the slurry surface occurred at ﬂuence...

arXiv 2022

[2] [13]

Laser cleaning of stone materials: an overview of current research,

J. Brand, A. Wain, A. V . Rode, S. Madden, P . King and L. Rapp, Femtosecond pulse laser cleaning of spray paints from heritage stone surfaces, Optics Express 30 17 (2022) 31122 [14] J. S. Pozo-Antonio, A. Papanikolaou, K. Melessanaki, T. Rivas and P . Pouli, Laser-Assisted Removal of GraHiti from Granite: Advantages of the Simultaneous Use of Two Wavelen...

work page 2022